Add a Start date field to the task edit sheet

The draft's start date was always today and nothing in the sheet could
change it. Surface it as a date input after Repeats, pre-filled from the
definition's rule on edit, and refuse to submit an empty date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/src/definitions/DefinitionsTab.test.tsx

@@ -1118,3 +1118,95 @@ describe("DefinitionsTab icon picker", () => {
     expect(rowIcon(screen.getByTestId("definition-20"))).toBeNull();
   });
 });
+
+/* ── Start date ─────────────────────────────────────────────────────── */
+
+function startDateInput(s: HTMLElement): HTMLInputElement {
+  return within(s).getByLabelText("Start date", { selector: "input" }) as HTMLInputElement;
+}
+
+describe("DefinitionsTab start date", () => {
+  beforeEach(() => {
+    sessionStorage.clear();
+    storeTokens({ access_token: "at-123", expires_in: 600 });
+  });
+
+  afterEach(() => {
+    cleanup();
+    vi.unstubAllGlobals();
+    vi.useRealTimers();
+  });
+
+  it("a new task defaults to today and POSTs the chosen start date", async () => {
+    vi.useFakeTimers({ toFake: ["Date"] });
+    vi.setSystemTime(new Date(2026, 8, 23, 9, 0)); // Wed 23 Sep 2026, local
+    await renderReady();
+
+    fireEvent.click(screen.getByRole("button", { name: "New" }));
+    const s = sheet();
+    const input = startDateInput(s);
+    expect(input.type).toBe("date");
+    expect(input.value).toBe("2026-09-23");
+
+    fireEvent.change(within(s).getByRole("textbox", { name: "Title" }), {
+      target: { value: "Feed the cat" },
+    });
+    fireEvent.click(within(s).getByRole("button", { name: /Assigned children/ }));
+    fireEvent.click(within(s).getByRole("checkbox", { name: "Declan" }));
+    fireEvent.click(within(s).getByRole("button", { name: "Evening" }));
+    fireEvent.change(input, { target: { value: "2026-10-15" } });
+
+    await act(async () => {
+      fireEvent.click(within(s).getByRole("button", { name: "Save changes" }));
+    });
+    expect(api.writes()).toHaveLength(1);
+    expect(api.writes()[0].method).toBe("POST");
+    expect((api.writes()[0].body as { rule: DefinitionRule }).rule.start_date).toBe("2026-10-15");
+  });
+
+  it("an edit pre-fills the definition's start date and PATCHes a changed one", async () => {
+    await renderReady();
+    fireEvent.click(screen.getByTestId("definition-10"));
+    const s = sheet();
+    const input = startDateInput(s);
+    expect(input.value).toBe("2026-01-01");
+
+    fireEvent.change(input, { target: { value: "2026-11-02" } });
+    await act(async () => {
+      fireEvent.click(within(s).getByRole("button", { name: "Save changes" }));
+    });
+    expect(api.writes()).toEqual([
+      expect.objectContaining({
+        method: "PATCH",
+        path: "/api/v1/admin/quest-definitions/10",
+        body: expect.objectContaining({ rule: rule({ start_date: "2026-11-02" }) }),
+      }),
+    ]);
+  });
+
+  it("the occurrence preview requests the chosen start date", async () => {
+    await renderReady();
+    fireEvent.click(screen.getByTestId("definition-10"));
+    const s = sheet();
+    await waitFor(() => expect(api.previews()).toHaveLength(1));
+
+    fireEvent.change(startDateInput(s), { target: { value: "2027-03-01" } });
+    await settleDebounce();
+    await waitFor(() => expect(api.previews()).toHaveLength(2));
+    expect((api.previews()[1].body as { rule: DefinitionRule }).rule.start_date).toBe(
+      "2027-03-01",
+    );
+  });
+
+  it("a cleared start date is not submitted", async () => {
+    await renderReady();
+    fireEvent.click(screen.getByTestId("definition-10"));
+    const s = sheet();
+    fireEvent.change(startDateInput(s), { target: { value: "" } });
+    await act(async () => {
+      fireEvent.click(within(s).getByRole("button", { name: "Save changes" }));
+    });
+    expect(api.writes()).toHaveLength(0);
+    expect(within(s).getByText("Pick a start date.")).toBeTruthy();
+  });
+});

admin/src/definitions/DefinitionsTab.tsx

@@ -177,6 +177,7 @@ function parseWhole(value: string, min: number, max: number): number | null {
 function buildRule(draft: Draft): DefinitionRule | string {
   const interval = parseWhole(draft.interval, 1, 999);
   if (interval === null) return "Interval must be a whole number of at least 1.";
+  if (draft.startDate === "") return "Pick a start date.";
   const rule: DefinitionRule = {
     rule_type: "daily",
     interval,
@@ -740,6 +741,16 @@ function EditSheet({
             <OccurrencePreview ruleKey={previewKey} />
           </Field>
 
+          <Field label="Start date" id="defs-label-start">
+            <input
+              className="defs-input"
+              type="date"
+              aria-labelledby="defs-label-start"
+              value={draft.startDate}
+              onChange={(e) => update({ startDate: e.target.value })}
+            />
+          </Field>
+
           <Field label="Due time" id="defs-label-due">
             {orderedWindows.length === 0 ? (
               <p className="defs-hint">Pick a window to set its due time.</p>