fix(documents): stop Backspace deleting a document, and show the key that does

A focused card deleted on Backspace as well as Delete. The timeline's
roving tabindex gives a card focus the moment someone tabs into the
grid, Backspace is the reflex "go back" key, and a card carries no
delete control at all — so the reflex press removed a medical document
with nothing but a transient undo toast behind it.

Delete alone now deletes. The key resolution moves into a pure helper
in vault-utils so the contract is testable without a render harness,
and the card stops keeping that contract in a code comment: the overlay
button carries aria-keyshortcuts, and a kbd chip appears while the card
holds focus. The label is per-locale (Entf / Suppr / Supr / Canc).

messages/de.json

@@ -8621,7 +8621,8 @@
       "uploading": "Wird hochgeladen…",
       "dismissFailed": "Fehlgeschlagenen Upload ausblenden",
       "processing": "Wird verarbeitet…",
-      "ready": "Bereit"
+      "ready": "Bereit",
+      "deleteKey": "Entf"
     },
     "error": {
       "fileTooLarge": "Diese Datei ist größer als das Limit von {maxSize}.",

messages/en.json

@@ -8621,7 +8621,8 @@
       "uploading": "Uploading…",
       "dismissFailed": "Dismiss failed upload",
       "processing": "Processing…",
-      "ready": "Ready"
+      "ready": "Ready",
+      "deleteKey": "Del"
     },
     "error": {
       "fileTooLarge": "This file is larger than the {maxSize} limit.",

messages/es.json

@@ -8621,7 +8621,8 @@
       "uploading": "Subiendo…",
       "dismissFailed": "Descartar subida fallida",
       "processing": "Procesando…",
-      "ready": "Listo"
+      "ready": "Listo",
+      "deleteKey": "Supr"
     },
     "error": {
       "fileTooLarge": "Este archivo supera el límite de {maxSize}.",

messages/fr.json

@@ -8621,7 +8621,8 @@
       "uploading": "Téléversement…",
       "dismissFailed": "Ignorer le téléversement échoué",
       "processing": "Traitement en cours…",
-      "ready": "Prêt"
+      "ready": "Prêt",
+      "deleteKey": "Suppr"
     },
     "error": {
       "fileTooLarge": "Ce fichier dépasse la limite de {maxSize}.",

messages/it.json

@@ -8621,7 +8621,8 @@
       "uploading": "Caricamento…",
       "dismissFailed": "Ignora caricamento non riuscito",
       "processing": "Elaborazione…",
-      "ready": "Pronto"
+      "ready": "Pronto",
+      "deleteKey": "Canc"
     },
     "error": {
       "fileTooLarge": "Questo file supera il limite di {maxSize}.",

messages/pl.json

@@ -8621,7 +8621,8 @@
       "uploading": "Przesyłanie…",
       "dismissFailed": "Odrzuć nieudane przesyłanie",
       "processing": "Przetwarzanie…",
-      "ready": "Gotowe"
+      "ready": "Gotowe",
+      "deleteKey": "Del"
     },
     "error": {
       "fileTooLarge": "Ten plik przekracza limit {maxSize}.",

src/components/documents/__tests__/document-card.test.tsx

@@ -65,6 +65,38 @@ describe("<DocumentCard>", () => {
     expect(html).not.toContain("Download only");
   });
 
+  it("announces and shows the Delete shortcut when the card is deletable", () => {
+    const html = render(
+      <DocumentCard
+        document={doc()}
+        selected={false}
+        onToggleSelected={noop}
+        onOpen={noop}
+        onDelete={noop}
+        highlighted={false}
+      />,
+    );
+    // The keyboard contract used to live only in a code comment. It is now
+    // announced to assistive tech and drawn while the card holds focus.
+    expect(html).toContain('aria-keyshortcuts="Delete"');
+    expect(html).toContain('data-slot="document-delete-hint"');
+    expect(html).toContain("Del");
+  });
+
+  it("offers no delete shortcut when the card carries no delete handler", () => {
+    const html = render(
+      <DocumentCard
+        document={doc()}
+        selected={false}
+        onToggleSelected={noop}
+        onOpen={noop}
+        highlighted={false}
+      />,
+    );
+    expect(html).not.toContain("aria-keyshortcuts");
+    expect(html).not.toContain('data-slot="document-delete-hint"');
+  });
+
   it("shows the download-only badge for attachment-class documents", () => {
     const html = render(
       <DocumentCard

src/components/documents/__tests__/vault-utils.test.ts

@@ -6,6 +6,7 @@ import {
   buildVaultListApiSearch,
   classifyUploadFailure,
   countActiveFilters,
+  documentCardKeyAction,
   documentDateKey,
   expandRangeSelection,
   formatBytes,
@@ -411,3 +412,22 @@ describe("isRecentlyUploaded / isDocumentProcessing / hasProcessingDocument", ()
     expect(hasProcessingDocument([], NOW)).toBe(false);
   });
 });
+
+describe("documentCardKeyAction", () => {
+  it("deletes on Delete and selects on Space", () => {
+    expect(documentCardKeyAction("Delete")).toBe("delete");
+    expect(documentCardKeyAction(" ")).toBe("select");
+  });
+
+  it("does NOT delete on Backspace", () => {
+    // The reflex "go back" key on a card that takes focus by default, with
+    // no other delete control and only a transient undo toast behind it.
+    expect(documentCardKeyAction("Backspace")).toBeNull();
+  });
+
+  it("ignores every other key", () => {
+    for (const key of ["Enter", "Escape", "a", "ArrowDown", "Del"]) {
+      expect(documentCardKeyAction(key)).toBeNull();
+    }
+  });
+});

src/components/documents/document-card.tsx

@@ -29,6 +29,7 @@ import type { InboundDocumentDto } from "@/lib/validations/inbound-documents";
 import { DOCUMENT_KIND_ICONS } from "./document-kind-meta";
 import type { UploadQueueItem } from "./use-document-upload";
 import {
+  documentCardKeyAction,
   documentDateKey,
   formatBytes,
   isDocumentProcessing,
@@ -241,7 +242,15 @@ export function DocumentCard({
           (the checkbox stays reachable via its own z-10). Keyboard contract
           (roving tabindex owned by the timeline): Enter opens (native
           click), Space toggles selection, Delete removes with undo. Touch:
-          press-and-hold selects instead of opening. */}
+          press-and-hold selects instead of opening.
+
+          Backspace is deliberately NOT a delete key here. It is the reflex
+          "go back" key, the roving tabindex hands a card focus the moment
+          someone tabs into the grid, and a card carries no other delete
+          control — so the reflex press destroyed a medical document with
+          nothing but a transient undo toast to catch it. Delete alone,
+          announced through `aria-keyshortcuts` and shown as a `<kbd>` chip
+          while the card holds focus. */}
       <button
         type="button"
         data-slot="document-open"
@@ -254,10 +263,11 @@ export function DocumentCard({
           onOpen(document.id);
         }}
         onKeyDown={(e) => {
-          if (e.key === " ") {
+          const action = documentCardKeyAction(e.key);
+          if (action === "select") {
             e.preventDefault();
             onToggleSelected(document.id, e.shiftKey);
-          } else if (e.key === "Delete" || e.key === "Backspace") {
+          } else if (action === "delete") {
             e.preventDefault();
             onDelete?.(document.id);
           }
@@ -279,8 +289,23 @@ export function DocumentCard({
           onPrefetch?.(document.id);
         }}
         aria-label={t("documents.card.openLabel", { title })}
+        aria-keyshortcuts={onDelete ? "Delete" : undefined}
         className="focus-visible:ring-ring/50 absolute inset-0 rounded-xl focus-visible:ring-[3px] focus-visible:outline-none"
       />
+      {/* The only visible affordance for deleting a single card. Shown while
+          the card holds focus (same `group-focus-within` trigger the checkbox
+          uses) and hidden from the accessibility tree, because
+          `aria-keyshortcuts` on the overlay button already announces the same
+          key. `pointer-events-none` keeps it out of the click target. */}
+      {onDelete ? (
+        <kbd
+          aria-hidden
+          data-slot="document-delete-hint"
+          className="border-border bg-background text-muted-foreground pointer-events-none absolute right-2 bottom-2 hidden rounded border px-1.5 font-mono text-xs opacity-0 transition-opacity group-focus-within:opacity-100 sm:inline-block"
+        >
+          {t("documents.card.deleteKey")}
+        </kbd>
+      ) : null}
     </Card>
   );
 }

src/components/documents/vault-utils.ts

@@ -416,3 +416,22 @@ export function parseUploadResponse(
   }
   return classifyUploadFailure(status, envelope?.meta);
 }
+
+/** What a keypress on a focused document card does. */
+export type DocumentCardKeyAction = "select" | "delete" | null;
+
+/**
+ * Resolve the action for a keypress on a focused vault card.
+ *
+ * `Backspace` is deliberately absent. The timeline's roving tabindex hands a
+ * card focus the moment someone tabs into the grid, `Backspace` is the reflex
+ * "go back" key, and a card carries no other delete control — so the reflex
+ * press deleted a medical document with nothing but a transient undo toast
+ * between the person and the loss. `Delete` alone deletes, and the card shows
+ * that key while it holds focus.
+ */
+export function documentCardKeyAction(key: string): DocumentCardKeyAction {
+  if (key === " ") return "select";
+  if (key === "Delete") return "delete";
+  return null;
+}