feat(workouts): read a blank start as now minus the duration

Refs #1085. Logging a workout right after finishing it meant changing the
start first, or the form refused it as ending in the future. The start field
now opens blank; left blank, the workout is saved as starting the duration
before now. A start the person enters is validated as before.

src/components/workouts/manual-workout-form.tsx

@@ -276,12 +276,18 @@ export function ManualWorkoutForm({
           value={draft.start}
           onChange={(value) => update("start", value)}
           max={wallClockNow(new Date(), timezone)}
-          required
-          aria-required="true"
           aria-invalid={!!errors.start || undefined}
-          aria-describedby={describedBy("start", `${ids.start}-error`)}
+          aria-describedby={[
+            `${ids.start}-hint`,
+            describedBy("start", `${ids.start}-error`),
+          ]
+            .filter(Boolean)
+            .join(" ")}
           data-testid="manual-workout-start"
         />
+        <p id={`${ids.start}-hint`} className="text-muted-foreground text-xs">
+          {t("insights.workouts.manual.startHint")}
+        </p>
         <FieldError id={`${ids.start}-error`} message={fieldError("start")} />
       </div>
 

src/lib/workouts/__tests__/manual-entry.test.ts

@@ -154,12 +154,31 @@ describe("buildManualWorkoutEntry — validation", () => {
     );
   });
 
-  it("asks for a start", () => {
-    expect(errorsOf({ start: "" }).start).toBe(
+  it("refuses a start it cannot read", () => {
+    expect(errorsOf({ start: "2026-09-15" }).start).toBe(
       "insights.workouts.manual.errors.startRequired",
     );
   });
 
+  it("reads a blank start as now minus the duration (#1085)", () => {
+    // NOW is 12:00Z; 45 minutes logged right after finishing.
+    const result = buildManualWorkoutEntry(draft({ start: "" }), CTX);
+    expect(result.ok).toBe(true);
+    expect(result.ok && result.entry.startedAt).toBe(
+      "2026-09-15T11:15:00.000Z",
+    );
+    expect(result.ok && result.entry.endedAt).toBe("2026-09-15T12:00:00.000Z");
+  });
+
+  it("never reports a blank start as ending in the future", () => {
+    const result = buildManualWorkoutEntry(
+      draft({ start: "", hours: "3", minutes: "0" }),
+      CTX,
+    );
+    expect(result.ok).toBe(true);
+    expect(result.ok && result.entry.endedAt).toBe("2026-09-15T12:00:00.000Z");
+  });
+
   it("refuses a zero duration", () => {
     expect(errorsOf({ hours: "", minutes: "" }).duration).toBe(
       "insights.workouts.manual.errors.durationRequired",
@@ -221,11 +240,11 @@ describe("buildManualWorkoutEntry — validation", () => {
 });
 
 describe("the form's defaults", () => {
-  it("opens on the current minute in the profile zone", () => {
+  it("opens with a blank start; the field caps at the current minute", () => {
     expect(wallClockNow(NOW, "Europe/Berlin")).toBe("2026-09-15T14:00");
     expect(emptyManualWorkoutDraft(NOW, "Asia/Seoul")).toMatchObject({
       sportType: "",
-      start: "2026-09-15T21:00",
+      start: "",
       hours: "",
       minutes: "",
     });

src/lib/workouts/manual-entry.ts

@@ -124,7 +124,9 @@ export function emptyManualWorkoutDraft(
 ): ManualWorkoutDraft {
   return {
     sportType: "",
-    start: wallClockNow(now, timezone),
+    // Blank on purpose (#1085): a blank start means "it just ended", so the
+    // workout is saved as starting `duration` before now.
+    start: "",
     hours: "",
     minutes: "",
     distance: "",
@@ -191,16 +193,17 @@ function manualWorkoutSchema(ctx: ManualWorkoutContext) {
         });
       }
 
-      const startedAt = startToUtc(d.start, ctx.timezone);
-      if (startedAt === null) {
+      const startBlank = d.start.trim() === "";
+      const startedAt = startBlank ? null : startToUtc(d.start, ctx.timezone);
+      if (!startBlank && startedAt === null) {
         issue.addIssue({
           code: "custom",
           path: ["start"],
           message: `${ERR}.startRequired`,
         });
       } else if (
-        startedAt.getTime() >
-        ctx.now.getTime() + FUTURE_TOLERANCE_MS
+        startedAt !== null &&
+        startedAt.getTime() > ctx.now.getTime() + FUTURE_TOLERANCE_MS
       ) {
         issue.addIssue({
           code: "custom",
@@ -236,8 +239,8 @@ function manualWorkoutSchema(ctx: ManualWorkoutContext) {
             message: `${ERR}.durationTooLong`,
           });
         } else if (
-          // The default start is "now", so a workout logged right after it
-          // finished would otherwise be stored as ending in the future.
+          // Only a start the person entered can push the end past now; a
+          // blank start is read as "now minus the duration".
           startedAt !== null &&
           startedAt.getTime() + sec * 1000 >
             ctx.now.getTime() + FUTURE_TOLERANCE_MS
@@ -306,10 +309,13 @@ export function buildManualWorkoutEntry(
   }
 
   // Every branch below was proven by the schema above.
-  const startedAt = startToUtc(draft.start, ctx.timezone)!;
   const durationSec =
     ((parseWhole(draft.hours) ?? 0) * 60 + (parseWhole(draft.minutes) ?? 0)) *
     60;
+  const startedAt =
+    draft.start.trim() === ""
+      ? new Date(ctx.now.getTime() - durationSec * 1000)
+      : startToUtc(draft.start, ctx.timezone)!;
   const distance = parseDecimal(draft.distance);
   const energy = parseDecimal(draft.energyKcal);