fix: stop repeated GitHub PR fetch warnings for non-GitHub repos

When opening a git repository without a GitHub remote, the server logs
were spammed with warnings every 5 seconds during worktree polling:

  WARN [Worktree] Failed to fetch GitHub PRs: Command failed: gh pr list
  ... no git remotes found

This happened because fetchGitHubPRs() ran `gh pr list` without first
checking if the project has a GitHub remote configured.

Changes:
- Add per-project cache for GitHub remote status with 5-minute TTL
- Check cache before attempting to fetch PRs, skip silently if no remote
- Add forceRefreshGitHub parameter to clear cache on manual refresh
- Pass forceRefreshGitHub when user clicks the refresh worktrees button

This allows users to add a GitHub remote and immediately detect it by
clicking the refresh button, while preventing log spam during normal
polling for projects without GitHub remotes.

apps/ui/src/lib/electron.ts

@@ -1596,10 +1596,15 @@ function createMockWorktreeAPI(): WorktreeAPI {
       return { success: true, worktrees: [] };
     },
 
-    listAll: async (projectPath: string, includeDetails?: boolean) => {
+    listAll: async (
+      projectPath: string,
+      includeDetails?: boolean,
+      forceRefreshGitHub?: boolean
+    ) => {
       console.log('[Mock] Listing all worktrees:', {
         projectPath,
         includeDetails,
+        forceRefreshGitHub,
       });
       return {
         success: true,