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

Sidebar (strikethrough on threads with missing directory):

Chat view (error banner + disabled composer):

Recovery (after restoring the folder and focusing the window):

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 in chat UI when a project directory is missing

• Adds a projects.checkDirectories WebSocket RPC method (ws.ts, wsServer.ts) that accepts a list of paths and returns which ones are missing or not directories.
• A new useProjectDirectoryCheck hook polls on mount, on project change, and on window focus, storing results in a new missingProjectCwds set in the app store.
• When a project directory is missing, ChatView disables the composer, shows a non-dismissible error banner, and blocks message sending.
• Thread titles in the Sidebar render with line-through and destructive color for missing directories.
• assertSupportedCodexCliVersion now distinguishes between a missing CLI binary and a missing project directory when ENOENT is returned.

^{Macroscope summarized 17f3f5a.}

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