fix: show clear error when project directory is missing

[chuks-qua]

What Changed

When a project folder is deleted or moved, the app now shows a clear "Project directory does not exist" error instead of the misleading "Codex CLI (codex) is not installed or not executable" message. The composer is disabled, sidebar threads show strikethrough styling, and everything auto-recovers when the folder is restored.

Why

Fixes #975. When a project directory no longer exists, spawnSync("codex", ["--version"], { cwd: deletedPath }) fails with ENOENT. The existing code assumed ENOENT always meant the binary was missing, producing a confusing error. Users had no way to understand the actual problem or recover from it.

Key Changes

• Server: Disambiguate ENOENT in assertSupportedCodexCliVersion by checking existsSync(input.cwd) in the error path (zero cost in happy path)
• WS endpoint: New projects.checkDirectories method that stats project cwds in parallel via Effect.forEach
• React hook: useProjectDirectoryCheck checks directories on mount and window focus (debounced 500ms), using shallow-compared cwd selectors to avoid unnecessary re-checks
• Sidebar: Threads under missing projects show line-through text-destructive/60 strikethrough
• ChatView: Non-dismissible error banner, disabled composer with helpful placeholder, blocked sending. All auto-clear when directory is restored.

UI Changes

Before

After

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Show clear error and disable chat input when project directory is missing

• Adds a projects.checkDirectories WebSocket RPC endpoint in wsServer.ts that stats each provided path and returns missing or non-directory cwds.
• Adds a useProjectDirectoryCheck hook that polls on mount, project changes, and window focus, storing results in a new missingProjectCwds set in the app store.
• In ChatView.tsx, blocks message sending and shows a non-dismissible ThreadErrorBanner when the active project directory is missing.
• In Sidebar.tsx, threads with missing project directories are shown with strikethrough and muted destructive color.
• Improves the CLI version check in codexAppServerManager.ts to distinguish between a missing cwd and a missing CLI binary on ENOENT failures.

^{Macroscope summarized 29fb16a.}

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[chuks-qua]

@juliusmarminge could you please authorize the ci, and check review please