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/http-api-client.ts

@@ -1724,8 +1724,8 @@ export class HttpApiClient implements ElectronAPI {
     getStatus: (projectPath: string, featureId: string) =>
       this.post('/api/worktree/status', { projectPath, featureId }),
     list: (projectPath: string) => this.post('/api/worktree/list', { projectPath }),
-    listAll: (projectPath: string, includeDetails?: boolean) =>
-      this.post('/api/worktree/list', { projectPath, includeDetails }),
+    listAll: (projectPath: string, includeDetails?: boolean, forceRefreshGitHub?: boolean) =>
+      this.post('/api/worktree/list', { projectPath, includeDetails, forceRefreshGitHub }),
     create: (projectPath: string, branchName: string, baseBranch?: string) =>
       this.post('/api/worktree/create', {
         projectPath,