feat: builtin miniapps + IM bot UX overhaul + disabled-model safeguards

[bobleer]

Summary

• Built-in MiniApps: bundles three apps (gomoku, daily-divination, regex-playground) and seeds them into miniapps_dir on first launch; reseeds on schema-version bump while preserving the user's storage.json.
• IM bot UX overhaul: introduces a shared locale (zh-CN / en-US BotStrings) and MenuView model under service/remote_connect/bot/, rewrites command_router on top of it, and updates Telegram / Feishu / WeChat adapters to render the same menu through their native primitives.
• Disabled-model safeguards: AIClientFactory now rejects requests routed to a disabled model with a clear error; SessionManager subscribes to config updates and auto-migrates active sessions back to auto when their bound model is disabled or removed, emitting a new SessionModelAutoMigrated event so the UI can refresh its model selector.
• FlowChat scroll stability: tightens VirtualMessageList / useFlowChatFollowOutput so streaming output no longer fights the user's scroll position; updated companion notes in FLOWCHAT_SCROLL_STABILITY.md.
• Misc: Tooltip a11y tweak, AgentAPI gains the new auto-migration hook, locale strings for new flows.

Test plan

Compilation has not been re-verified locally — please run pnpm run desktop:dev (or cargo check -p bitfun-core) before merging if you want a clean build signal.

1. Built-in MiniApps

• Launch desktop app on a fresh profile (or remove <appdata>/miniapps/builtin-*).
• Open the MiniApp gallery and confirm three new entries appear: 五子棋 / Gomoku, 每日占卜, Regex Playground.
• Open each one, interact briefly, then restart the app and confirm:
  • the apps still appear (no duplicate seeding),
  • any state stored via storage.json (e.g. divination cards, regex input) survives restart.
• Manually edit <appdata>/miniapps/builtin-gomoku/.builtin-version to 0, restart, and confirm the source files are rewritten but storage.json is untouched.

2. IM bot UX (covers Telegram / Feishu / WeChat)

For each platform you have configured:

• Send /help (or the platform's equivalent text) and confirm the new menu renders with native controls (Telegram inline buttons, Feishu interactive card, WeChat numbered list).
• Switch app language between zh-CN and en-US, trigger the menu again, and confirm all labels/replies localize correctly.
• Walk through one full command flow (e.g. select an agent → ask a question → receive answer) and confirm there are no regressions vs the previous router behavior.
• Confirm WeChat numeric shortcuts (1, 2, …) still resolve to the correct command.

3. Disabled / removed models

• Open settings → AI models, disable the model that an active chat session is currently using, and save.
  • The chat should immediately fall back to auto (model selector refreshes; a notice may appear in the chat).
  • Sending a new message should succeed via the auto-routed model.
• Manually invoke a disabled model (e.g. by editing the request) and confirm the backend returns a clear Model '...' is currently disabled error instead of silently failing.
• Delete a model that a session is bound to and confirm the same auto-migration happens.

4. FlowChat scroll stability

• Start a long streaming agent reply and scroll up while output is still streaming — the viewport should stay where you left it (no auto-jump-to-bottom).
• Scroll back to the bottom and confirm follow-output resumes automatically.
• Resize the window mid-stream and confirm no flicker / scroll jump.

5. Smoke

• Tooltip still renders correctly on hover/focus across the app (no a11y regressions).
• App language switch (zh-CN ↔ en-US) updates new strings in flow-chat without restart.