Chat mode, phase 2: local history, answer actions, motion polish

- Conversations in a versioned IndexedDB store (freezmo-chat v1, zod
  validated with the same schema backup/sync use, capped at 100 x 300).
  Settled turns persist (question always; answer when produced, partial
  stops included); ?c=<id> keeps reloads on the conversation via
  history.replaceState (no router round-trip mid-chat).
- History UI: desktop column, phone sheet (spring, backdrop, Escape),
  auto titles from the first question, inline rename, delete with an
  Undo toast, diacritic-folding search over titles and message text.
  Scope and model are remembered per conversation; history is readable
  offline.
- Assistant actions: Turn into flashcards (New deck with the answer as
  notes, landing on the options step - deck seed gained step) and Save
  to notes (appends to the single scoped deck, trimmed at 40k).
- Chats travel in the backup export (preview counts them) and in the
  encrypted sync payload; still never the key.
- Motion from lib/motion.ts: message fade-up, sheet springs for the
  model/context/history sheets, 150ms composer height transition, all
  skipped under reduced motion.
- Tests: chat suite 63 -> 89 checks (history, reload, rename/search/
  delete/undo, actions, export, two-device sync round-trip, offline,
  desktop panel). Full set: 12/12 suites, 513 checks. SW -> freezmo-v33.

Also: tests/model-compare.mjs benchmark (diagnostic) for deciding the
default text model on numbers; defaults unchanged.

lib/backup.ts

@@ -12,11 +12,12 @@
 import { z } from "zod";
 
 import { applyTheme } from "@/components/ThemeToggle";
+import { chatHistoryStore, ensureChatsHydrated, replaceChats } from "./chat-store";
 import { replaceFolders, folderStore } from "./folders";
 import { homeworkStore, replaceHomework } from "./homework";
 import { ensureLearningHydrated, learningStore, replaceLearning } from "./history";
 import { setLocale, t } from "./i18n";
-import { folderSchema, deckSchema, homeworkRecordSchema, answerRecordSchema, srsRecordSchema } from "./schemas";
+import { folderSchema, deckSchema, homeworkRecordSchema, answerRecordSchema, srsRecordSchema, chatConversationSchema } from "./schemas";
 import { saveSettings, settingsStore } from "./settings";
 import { deckStore, replaceDecks } from "./storage";
 
@@ -36,6 +37,8 @@ export const backupDataSchema = z.object({
   /** 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(),
+  /** Chat conversations (IndexedDB). Optional for old backups. */
+  chats: z.array(chatConversationSchema).max(100).optional(),
 });
 
 export const backupSchema = z.object({
@@ -54,6 +57,7 @@ export type BackupFile = z.infer<typeof backupSchema>;
 
 export async function buildBackup(): Promise<BackupFile> {
   await ensureLearningHydrated();
+  await ensureChatsHydrated();
   return {
     app: "freezmo",
     kind: "backup",
@@ -65,6 +69,7 @@ export async function buildBackup(): Promise<BackupFile> {
       homework: homeworkStore.getSnapshot(),
       history: learningStore.getAnswersSnapshot(),
       srs: learningStore.getSrsSnapshot(),
+      chats: chatHistoryStore.getSnapshot(),
       settings: {
         haptics: settingsStore.getSnapshot().haptics,
         dailyGoal: settingsStore.getSnapshot().dailyGoal,
@@ -138,6 +143,7 @@ export type ImportPreview = {
   decks: ImportCounts;
   folders: ImportCounts;
   homework: ImportCounts;
+  chats: ImportCounts;
 };
 
 export function previewImport(backup: BackupFile): ImportPreview {
@@ -145,6 +151,7 @@ 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),
+    chats: countMerge(chatHistoryStore.getSnapshot(), backup.data.chats ?? []),
   };
 }
 
@@ -184,6 +191,13 @@ export async function applyImport(
     await replaceLearning(answers, [...srsByKey.values()]);
   }
 
+  // Chat conversations: merge by id (newest wins) or replace wholesale.
+  await ensureChatsHydrated();
+  const incomingChats = data.chats ?? [];
+  if (incomingChats.length > 0) {
+    replaceChats(mode === "replace" ? incomingChats : mergeById(chatHistoryStore.getSnapshot(), incomingChats));
+  }
+
   const settings = data.settings;
   if (settings) {
     if (typeof settings.haptics === "boolean") saveSettings({ haptics: settings.haptics });