Install probe: use the saved app pet glyph as launcher art

[lannbot]

Changes

• Render the saved app pet glyph on the user's hue at 192 and 512 pixels.
• Keep the working blob manifest; cache PNGs locally and serve only launcher-icon URLs through a service worker.
• Retain static icons when the painted path is unavailable. No offline shell caching.
• Document that cached launcher art is unsealed and that Android WebAPK installation remains a real-device probe.

Verification

• just ci passed.
• just e2e: 23/23 passed, including saved-glyph pixels, persistence, HTTP-vs-worker behavior, static fallback, and a Pages-style subpath.

Automerge will be armed. Browser tests do not establish Android installation success.