Fix diff refresh loop and idle work

[parsakhaz]

Summary

• stabilize Diff panel refreshes so git status updates do not destructively reload while active
• preserve visible diff content during soft refreshes and ignore stale async responses
• stop inactive resource/browser work that was hurting battery and pane switching

Testing

• pnpm --filter frontend typecheck
• pnpm --filter frontend lint
• pnpm --filter main typecheck
• pnpm --filter main lint
• pre-commit hook: pnpm run -r typecheck && pnpm run -r lint