fix(desktop): recover from missing or inaccessible project folders - #50952

Issue for this PR

Closes #50951

Depends on #51171. This branch is stacked on it, so the fix(server) commit here belongs to #51171. Review only the fix(desktop) commit. Once #51171 merges, this diff will show only the app and Desktop changes.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The server now explains unavailable project folders: LocationNotFoundError (404) from #52668, plus LocationPermissionDeniedError (403) and the location.get re-check from #51171. Desktop still shows broken screens or a generic error page for both. This PR uses those errors to explain the problem and offer recovery.

• Session and draft tabs: when the folder is missing or blocked, the tab shows a panel instead of a composer whose requests all fail.
  • Session tab: Move session (uses the existing move API, which does not need the old folder), Try again, Close project / Close tab.
  • Draft tab: Choose directory, Try again, Close project / Close tab.
  • Close project only appears when the unavailable folder is a saved root project, so a missing worktree never closes its parent.
• Folders lost while OpenCode is running: the background server keeps a project's services cached, and the client caches its details too. When a cached project or restored tab is entered, Desktop calls location.get again, which re-checks the folder (fix(server): report location permission errors #51171), so the panel also appears for a folder deleted or renamed mid-session.
• Permission wording: a blocked folder says the account needs read access. When the server runs on this Mac, it instead points to System Settings > Privacy & Security > Files & Folders (or Full Disk Access). It also notes that a terminal-started OpenCode needs the terminal allowed, and that opencode service restart may be needed.
• Adding a project: a missing or blocked folder shows a persistent toast. A missing folder also gets Remove project. Other failures keep the current quiet behavior.
• Error page: a saved route that fails this way explains the folder problem instead of "HTTP 404/403", and Desktop forgets that route so Restart returns to the project list.
• First launch on macOS: if macOS blocks creating the default project in Documents, onboarding still completes (so the declined privacy prompt is not repeated on every launch) and explains how to grant access. This is detected locally in the main process.

How did you verify your code works?

• bun run check from the repository root.
• packages/app: bun run test:unit passes, including a case that recognizes both server errors (direct and SDK-wrapped), uses the macOS wording only when asked, and ignores unrelated errors.
• packages/desktop: default-project tests cover creation and the denied-then-restored case.
• Session entry benchmark (session-entry-benchmark.spec.ts, cold session from Home, 5 runs each), since entering a cached project now makes one extra location.get: stable time 282–394 ms on v2, 250–301 ms with this PR.

Screenshots / recordings

Server-side before/after recordings for both cases are on #51171. Desktop panel recordings are not captured yet; they require an unavailable local project folder.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR