feat(web): add manual context compaction

[beilo]

Summary

Add a manual context-compaction action to the live chat UI.

• Add a Compact context button to the context-usage popover.
• Accept /compact and /compact <instructions> in the composer.
• Route both entry points through POST /api/compact to the pi RPC worker.
• Reject compaction while the session worker is busy and reload the session after success.
• Reset context-usage aggregation at the compaction boundary.

Related issue

Related to #41 — Support /compact command through RPC.

This PR covers the core button, composer command, and RPC path. The keyboard shortcut and dedicated Playwright E2E case requested in the issue discussion are not included yet.

Type of change

• feat — new feature
• test — adding or updating tests

Live vs. Export

• Not applicable — this PR doesn't touch session rendering
• Considered both the live app and the export snapshot
• Kept internal/ui/embedded/ in sync with web/src/session/ changes
• No live-only chrome (Vite scripts, active composer, SSE/API) leaked into export

The new action is live-app-only. Export rendering does not expose the composer or compact endpoint.

Testing

• make check
• make build
• Frontend unit tests cover command parsing, API calls, popover behavior, and context reset.
• Go tests cover the HTTP, worker, and RPC paths.
• Browser UI verification not run.