perf: speed up web startup and reduce route and preview work

[davidmckayv]

Desktop launches rebuilt the web app every time, sign-in loaded the entire route bundle, and hidden computer previews kept processing images. This reuses valid builds, groups lazily loaded route components, and pauses hidden preview image work while retaining bot/control connections.

• Build reuse invalidates on source, lockfile, version, Vite environment, and tenant inputs. Startup still regenerates tenant config before checking the cache; missing or invalid cache metadata rebuilds.
• Route groups preserve existing lazy diagram/syntax imports. Dependencies remain under Rollup's normal chunking to avoid initialization cycles.
• Hidden LiveScreen retains only the latest encoded frame and draws it on return, even without a new incoming frame. Screenshot previews pause offscreen/in hidden tabs; control polling continues.

Measured on macOS with Bun 1.3.14:

Check	Result
Warm web-process startup	104 ms median, versus 10–12 seconds for a rebuild; three warm samples
Sign-in JavaScript	3,193,056 → 725,746 bytes; same-source production builds
Chat JavaScript	3,193,056 → 2,873,498 bytes, across 13 scripts; naive automatic splitting needed 78
Hidden previews	0 image decodes/draws/screenshot polls in two 15-second samples per component; controls and resume verified

Startup timing excludes desktop/Docker/API startup. Route measurements use controlled API responses and uncompressed loopback serving. LiveScreen retains its socket/network traffic while hidden. Separate real-backend validation used LangGraph to navigate Example.com, click through to IANA, and render a bar chart; the screenshot and successful tool results were inspected. The admin credentials route also rendered without JavaScript errors. Visual evidence is attached to FOR-310.

Validation: 837 tests passed, format, lint, explicit type checks (including startup scripts), production build, frozen lockfile install, and actionlint. Added macOS/Windows CI startup tests; Linux remains covered by the full CI suite. Real branding-change/reuse/default-restoration checks verified the compiled output.

Fixes FOR-310

[linear-code]

FOR-310