feat(chat): token stats — tok/s, TTFT footer stat with settings toggle

[1337hero]

Closes #35

What

Optional per-message token stats footer on assistant messages: tokens/sec, TTFT, and token counts. Off by default via a new Settings toggle — when disabled, nothing renders (zero layout shift).

How

• Server (server/src/routes/chats.js): attaches {usage} as message metadata on the finish part via toUIMessageStreamResponse({ messageMetadata }).
• Client timing (useChatStream.js): TTFT measured send → first assistant text delta; duration measured client-side. Client composes metadata.stats from usage + timing so timing is measured where it happens.
• tok/s = outputTokens / (duration − TTFT) — generation rate, excluding TTFT.
• Persistence: stats ride the existing message-metadata path; messageShape.js now carries metadata through toCanonicalMessage, so stats survive reload.
• Toggle: showTokenStats in useThemeStore (localStorage, per-browser), matching the showCodeLineNumbers pattern.

Verification

• bun run test frontend: 52 pass, incl. 7 new tokenStats.test.js tests (null guards, tok/s math, zero-generation guard, formatting)
• bun test server: 420 pass
• bun run build, bun run format clean

Decisions to ratify

• Toggle is client-side (localStorage) rather than server app settings — matches existing display-preference pattern.
• Not yet checked against a live Ollama vs cloud provider (issue's acceptance check) — worth a manual pass before merge.