Fix UI recovery false positives on slow loads

[ymichael]

Root cause:
The server injected the UI-source recovery watchdog into shipped/default index.html. On slow or stalled first loads, #root could stay empty long enough for the watchdog to show the UI-source recovery bar even when UI forking was disabled and no fork existed. For active UI forks, the old watchdog also auto-reverted after a fixed timeout, which could interrupt a slow-but-eventually-successful customized UI load.

Fix:
Keep the live-reload shim on all served HTML, but enable recovery behavior only when the server is actually serving an active UI fork dist. Active-fork recovery is now manual-only: slow loads, script load failures, runtime errors, or unhandled rejections before mount show the Revert to stable banner, but no timer calls /api/v1/ui/prod. If the custom UI eventually mounts, the banner is removed. Clicking Revert to stable still posts /api/v1/ui/prod and reloads.

Regression coverage added/confirmed:

• Shipped/default UI gets the shim for live reload, but the recovery watchdog remains disabled and does not show the banner after timeout or script-error signals.
• Active customized UI shows the recovery banner when it fails to mount.
• Slow customized UI can keep loading; if #root eventually mounts, the banner disappears and no revert request is sent.
• Clicking Revert to stable calls POST /api/v1/ui/prod and reloads.
• Live reload on system ui-reloaded still reloads for both shipped/default HTML and active UI-source HTML.
• Static serving still marks shipped HTML recovery disabled and active fork HTML recovery enabled.

Verification:

• pnpm exec turbo run test --filter=@bb/server -- src/services/ui-source/ui-source.test.ts test/app/static-cache.test.ts
• pnpm exec turbo run typecheck --filter=@bb/server
• pnpm exec turbo run build --filter=@bb/server
• Merge-tree conflict check against latest origin/main is clean.