fix(git): restore non-repo fallback and refine repo preflight errors

Reintroduce a local not-a-git-repo fallback state in the git log panel so non-repo errors still render the dedicated empty state when workspace state streaming is degraded.

Improve git repository preflight classification by distinguishing missing paths, permission issues, and non-directory targets before checking .git presence.

Add not_a_git_repository to the frontend AppErrorCode union for explicit typed handling.

src/lib/types.ts

@@ -11,6 +11,7 @@ export type AppErrorCode =
   | "configuration_missing"
   | "configuration_invalid"
   | "not_found"
+  | "not_a_git_repository"
   | "already_exists"
   | "permission_denied"
   | "dependency_missing"