feat(web): sleep timer upgrades — undo toast, chain mode, clock stop, wind-down dim (plan §17)

- Undo: a genuine sleep fire now toasts with a Resume action (toast.ts
  grows an optional action button; acting dismisses).
- Chain: 'End of chapter, then 15 min' — leaving the chapter starts a
  15-min countdown instead of fading (SleepState.followUpMinutes).
- Clock: the popover gains a time input + Arm — armClockTime resolves
  HH:MM to its next occurrence and reuses the minutes machinery.
- Dim: the ticker eases a pointer-events-none overlay toward 35% black
  across a countdown's final 30s; stop/finish resets it.
(Fade-out and countdown labels already existed — §3 built them.)

web/src/audio-sleep.test.ts

@@ -3,6 +3,7 @@ import { describe, expect, it } from "vitest";
 import {
   SLEEP_CHOICES,
   SLEEP_OFF,
+  armClockTime,
   armEndOfChapter,
   armMinutes,
   fadeVolume,
@@ -77,13 +78,69 @@ describe("formatRemaining / sleepLabel", () => {
   it("labels every mode", () => {
     expect(sleepLabel(SLEEP_OFF, 0)).toBe("Off");
     expect(sleepLabel(armEndOfChapter(2), 0)).toBe("End of chapter");
-    expect(sleepLabel({ mode: "fading", endsAtMs: null, chapterIndex: null }, 0)).toBe("Fading");
+    expect(
+      sleepLabel({ mode: "fading", endsAtMs: null, chapterIndex: null, followUpMinutes: null }, 0),
+    ).toBe("Fading");
     expect(sleepLabel(armMinutes(1000, 5), 1000 + 65_000)).toBe("3:55");
   });
 });
 
+describe("armClockTime (§17 stop-at-clock)", () => {
+  it("arms a minutes-state deadline at the next occurrence of HH:MM", () => {
+    // 10:00 local → 23:00 same day.
+    const tenAm = new Date();
+    tenAm.setHours(10, 0, 0, 0);
+    const state = armClockTime(tenAm.getTime(), "23:00");
+    expect(state?.mode).toBe("minutes");
+    const expected = new Date(tenAm);
+    expected.setHours(23, 0, 0, 0);
+    expect(state?.endsAtMs).toBe(expected.getTime());
+    // Label is the ordinary countdown machinery (12h59m → 779:00).
+    expect(sleepLabel(state!, tenAm.getTime() + 60_000)).toBe("779:00");
+  });
+
+  it("wraps to tomorrow when the time already passed", () => {
+    const noon = new Date();
+    noon.setHours(12, 0, 0, 0);
+    const state = armClockTime(noon.getTime(), "09:15");
+    const expected = new Date(noon);
+    expected.setDate(expected.getDate() + 1);
+    expected.setHours(9, 15, 0, 0);
+    expect(state?.endsAtMs).toBe(expected.getTime());
+  });
+
+  it("rejects unparseable and out-of-range input", () => {
+    const now = Date.now();
+    expect(armClockTime(now, "nope")).toBeNull();
+    expect(armClockTime(now, "24:00")).toBeNull();
+    expect(armClockTime(now, "10:61")).toBeNull();
+  });
+});
+
+describe("§17 chain: end-of-chapter with a follow-up countdown", () => {
+  it("arms the follow-up on the state; the chapter fire contract is unchanged", () => {
+    const state = armEndOfChapter(2, 15);
+    expect(state.followUpMinutes).toBe(15);
+    // The sync contract (any chapter change fires) is the same — the
+    // component converts to armMinutes instead of fading.
+    expect(sleepChapterFired(state, 3)).toBe(true);
+    expect(sleepChapterFired(state, 1)).toBe(true);
+    expect(sleepChapterFired(state, 2)).toBe(false);
+  });
+
+  it("labels the chapter phase with the follow-up length", () => {
+    expect(sleepLabel(armEndOfChapter(2, 15), 0)).toBe("Ch + 15m");
+    expect(sleepLabel(armEndOfChapter(2), 0)).toBe("End of chapter");
+  });
+
+  it("offers the chain row in the dropdown vocabulary", () => {
+    const chain = SLEEP_CHOICES.find((c) => c.key === "eoch15");
+    expect(chain).toMatchObject({ mode: "end-of-chapter", minutes: 15, chain: true });
+  });
+});
+
 describe("SLEEP_CHOICES", () => {
-  it("is the agreed option list: off, 5/10/15/30/45/60, end-of-chapter", () => {
+  it("is the agreed option list: off, 5/10/15/30/45/60, end-of-chapter, chain", () => {
     expect(SLEEP_CHOICES.map((c) => c.key)).toEqual([
       "off",
       "5",
@@ -93,6 +150,7 @@ describe("SLEEP_CHOICES", () => {
       "45",
       "60",
       "eoch",
+      "eoch15",
     ]);
     expect(SLEEP_CHOICES.filter((c) => c.mode === "minutes").map((c) => c.minutes)).toEqual([
       5, 10, 15, 30, 45, 60,