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).

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();
+    }
+  });
+});