Allow retrying recoverable desktop startup failures

[bb-slop-cop]

Human comments

What was wrong

Recoverable desktop startup failures rendered a static local error view with no action path back to the selected server target. The remote loader stopped after its first rejected navigation even though the existing target application flow could perform the required reconnect or reauthentication. The direct reproduction and root-cause report is https://get-bb.github.io/reports/issues/3223.html.

What changed

Startup errors now carry an explicit retryable flag. Recoverable remote-load, Connect-authentication, and local-attach failures render a Try again button, while fatal process, port-conflict, and shutdown errors remain non-retryable. The preload sends a fixed payload-free IPC event; the main process validates its payload, sender window, and local-view URL before showing the loading state and reapplying the selected target. This changes no host wire protocol, CLI, configuration, dependency, or public plugin API.

How you verified

• Focused regression: failed before the production change because the recoverable error HTML had no retry control; passes after the change with fatal errors still lacking the control.
• pnpm exec turbo run typecheck --filter=@bb/desktop
• pnpm exec turbo run test --filter=@bb/desktop — 39 files passed, 294 tests passed, 1 skipped.
• pnpm exec turbo run build --filter=@bb/desktop
• pnpm exec oxfmt --check on all five changed files.
• Browser-rendered the generated recoverable error view and verified a visible BUTTON labeled Try again.
• git diff --check
• Changed-line count: 95 total text lines (94 additions, 1 deletion) across five files.

Fixes #3223

AGENT GENERATED