v0.21.1 — desktop app installs as a PWA

The Chimera Desktop app is now installable to your desktop — no Electron, no Tauri, no Rust.

With `chimera app` running, open it in Chrome or Edge and click Install (the icon in the address bar, or ⋮ → Install Chimera). It then opens in its own window with a taskbar/dock icon — an app-like experience over the same local backend.

How

• A web app manifest (standalone display, SVG app icon, theme color) + a minimal service worker that stale-while-revalidates the static shell for instant startup. The SW never intercepts /api (or any POST), so the SSE chat stream is completely untouched.
• The backend now serves the .webmanifest with the correct application/manifest+json type.

Verified live: the service worker registers and activates at scope /, and the manifest validates (name Chimera, standalone). Gate: 1480 passed / 4 skipped, mypy + ruff green; frontend build clean.

Upgrade: `pip install --upgrade 'chimera-agent[desktop]'` then rebuild the UI and `chimera app` — the Install option appears in the browser.