fix: refresh GitHub status on activity

[AmethystLiang]

Summary

• Refresh GitHub data (PR/issue status) on activity: triggers refresh when switching worktrees (setActiveWorktree) and when the window regains focus (visibilitychange)
• New store actions: refreshAllGitHub invalidates all cache entries and re-fetches; refreshGitHubForWorktree does the same for a single worktree
• Background poll for issues: 5-minute interval fallback in WorktreeCard ensures issue data stays fresh even without user interaction
• Fix: skip redundant GitHub refresh when re-clicking the already-active worktree

Test plan

• Switch between worktrees and verify PR/issue badges update
• Tab away from the app, change a PR/issue status on GitHub, tab back — verify the status updates
• Click the same worktree card repeatedly — verify no unnecessary API calls in network tab
• Leave app open on a worktree with a linked issue for >5 minutes — verify issue data refreshes via background poll

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