Podcast mode: notes read aloud with a lyric-style word highlight

The third segment of the note-powered wizards (Study deck / Homework /
Podcast) takes the same intake - photos, documents, pasted text, the share
target - and turns it into a spoken episode.

- prompts: PODCAST_SYSTEM_PROMPT writes for the ear (plain prose, no markdown,
  no visual references), chaptered into 3-8 sections around a length target.
- /api/podcast mirrors /api/generate's plumbing (rate limit, model gate,
  provider keys, heartbeats, the 2x deadline, the optional vision pass).
- storage: frezmo.podcasts.v1, zod-validated, included in the backup and the
  encrypted sync payload; a new library tab lists episodes with listen /
  rename / delete-with-undo, and the wizard switch deep-links in.
- player: the transcript is word-wrapped spans and the word under the voice
  rides a lyric-style highlight that keeps itself centred. Two clocks: exact
  onboundary events where the browser reports them, a per-word length timer
  where it does not (iOS Safari); the first boundary retires the timer.
  Pause = cancel + re-speak from the word, which makes resume, chapter jumps
  and tap-any-word-to-seek the same operation; the position saves on pause,
  page-hide and unmount, so reopening resumes. Speed rebuilds the engine and
  keeps playing; the Aero soundtrack pauses itself when the voice starts; a
  browser with no voice gets an honest note and a readable transcript.
- the script sits on a glass pane (plain paragraphs over Aero's grass failed
  contrast; the scan now covers the player too).

tests/podcast-test.mjs (17th suite) stubs the model and the speech engine and
drives the highlight by hand: 26 checks. All suites green, contrast clean.
Version 0.7.3, SW v95.

lib/backup.ts

@@ -15,9 +15,10 @@ import { applyTheme, type Theme } from "./theme";
 import { chatHistoryStore, ensureChatsHydrated, replaceChats } from "./chat-store";
 import { replaceFolders, folderStore } from "./folders";
 import { homeworkStore, replaceHomework } from "./homework";
+import { podcastStore, replacePodcasts } from "./podcasts";
 import { ensureLearningHydrated, learningStore, replaceLearning } from "./history";
 import { setLocale, t } from "./i18n";
-import { folderSchema, deckSchema, homeworkRecordSchema, answerRecordSchema, srsRecordSchema, chatConversationSchema } from "./schemas";
+import { folderSchema, deckSchema, homeworkRecordSchema, podcastSchema, answerRecordSchema, srsRecordSchema, chatConversationSchema } from "./schemas";
 import { saveSettings, settingsStore } from "./settings";
 import { deckStore, replaceDecks } from "./storage";
 
@@ -34,6 +35,8 @@ export const backupDataSchema = z.object({
   decks: z.array(deckSchema).max(2000),
   folders: z.array(folderSchema).max(500),
   homework: z.array(homeworkRecordSchema).max(2000),
+  /** Podcast episodes. Optional for old backups. */
+  podcasts: z.array(podcastSchema).max(2000).optional(),
   /** Answer log and SRS schedule (IndexedDB). Optional for old backups. */
   history: z.array(answerRecordSchema).max(20_000).optional(),
   srs: z.array(srsRecordSchema).max(50_000).optional(),
@@ -67,6 +70,7 @@ export async function buildBackup(): Promise<BackupFile> {
       decks: deckStore.getSnapshot(),
       folders: folderStore.getSnapshot(),
       homework: homeworkStore.getSnapshot(),
+      podcasts: podcastStore.getSnapshot(),
       history: learningStore.getAnswersSnapshot(),
       srs: learningStore.getSrsSnapshot(),
       chats: chatHistoryStore.getSnapshot(),
@@ -143,6 +147,7 @@ export type ImportPreview = {
   decks: ImportCounts;
   folders: ImportCounts;
   homework: ImportCounts;
+  podcasts: ImportCounts;
   chats: ImportCounts;
 };
 
@@ -151,6 +156,7 @@ export function previewImport(backup: BackupFile): ImportPreview {
     decks: countMerge(deckStore.getSnapshot(), backup.data.decks),
     folders: countMerge(folderStore.getSnapshot(), backup.data.folders),
     homework: countMerge(homeworkStore.getSnapshot(), backup.data.homework),
+    podcasts: countMerge(podcastStore.getSnapshot(), backup.data.podcasts ?? []),
     chats: countMerge(chatHistoryStore.getSnapshot(), backup.data.chats ?? []),
   };
 }
@@ -167,12 +173,15 @@ export async function applyImport(
     mode === "replace" ? data.folders : mergeById(folderStore.getSnapshot(), data.folders);
   const homework =
     mode === "replace" ? data.homework : mergeById(homeworkStore.getSnapshot(), data.homework);
+  const podcasts =
+    mode === "replace" ? data.podcasts ?? [] : mergeById(podcastStore.getSnapshot(), data.podcasts ?? []);
 
   // Folders first so imported decks can point at them. Importing is a real
   // local change, so it also marks sync dirty (via the stores' write()).
   replaceFolders(folders);
   replaceDecks(decks);
   replaceHomework(homework);
+  replacePodcasts(podcasts);
 
   // Answer log + schedule: merge by id (answers) and by key (schedules), or
   // replace wholesale when mode === "replace".