feat(chat): give a notebook many named chat sessions (#97)

[mrsibe]

What does this PR do?

Gives a notebook many named chat sessions instead of exactly one, and turns the
silent automatic context rollover into a visible, navigable archive boundary.

Why?

A notebook used to hold one conversation, and SessionAutoSwitchService rolled it
over silently when it approached the context budget. Every topic shared one
thread and its context; the reader could not start a separate line of inquiry;
and the rollover was a backend event they could neither see nor navigate. This
implements #97.

Related issue

Fixes #97

What changed?

• Schema (additive migration 0022_concerned_spyke): chat_sessions gains
  last_opened_at (which session the notebook returns to) and title_is_auto
  (a placeholder title the first message may replace).
• Queries: getMostRecentSessionByNotebook replaces
  getActiveSessionByNotebook (many sessions can be active, so status can no
  longer identify one); touchSession, deriveSessionTitleIfAuto,
  searchMessagesInNotebook, countSessionsByNotebook, and use-ordered session
  listing.
• IPC/preload: touch-session and search-messages; send-message names a
  placeholder session from its first user message.
• Auto-switch: archives the old session, creates a continuation with
  parentSessionId pointing back at it, and drops the silent "context optimized"
  system message. The rollover stays visible through the data, not a one-shot
  toast.
• Renderer: SessionSwitcher in the chat panel header (list, + New chat,
  rename, confirmed delete, message search across sessions, archived badge) and
  SessionContinuationNotice (the archive boundary with an "open archived chat"
  action).
• i18n: new strings in en-US and zh-CN.

How was this tested?

• CI Verify is green on Linux, macOS and Windows — typecheck, design guard,
  unit tests, electron-builder --dir, and smoke:packaged (which runs the real
  packaged app and therefore applies migration 0022).
• npm test — 478 pass, including new test/sessionTitle.test.ts and the
  updated automatic-switch renderer test.
• npx electron-vite build — bundles main, preload and renderer.

The manual workflow from the issue (three sessions, restart, confirm histories,
force a rollover) is covered by the packaged smoke run only at startup; a full
interactive pass is still worth doing before release.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes (CI).
• npm run build passes (CI).
• I have tested the affected user workflow (packaged smoke run; see note).
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable
• npm run build:unpack passes (CI).
• npm run smoke:packaged passes (CI).