fix(desktop): pause background IPC polls when window is hidden (#1079)

[xielixing]

Summary

Gate frontend polling intervals on document.visibilityState so IPC round-trips stop when the app window is backgrounded, eliminating the WebView2 subprocess CPU spike described in #1079.

Root Cause

Several frontend pollers run unconditionally for the entire app lifetime, issuing Tauri IPC calls even when the window is hidden and idle:

Poller	Interval	IPC calls per poll
DispatchJobObserver	1800 ms	dispatch_list_jobs + 1× dispatch_status per job
CodeEditor file-sync	1000 ms	file modification check
MarkdownEditor file-sync	1000 ms	file modification check
BackgroundCommandOutputPanel	1000 ms	background command output read

None checked document.visibilityState, so WebView2 kept processing IPC round-trips at full rate while backgrounded.

Fix

• DispatchJobObserver.ts: Early-return in run() when visibilityState === 'hidden'. The visibilitychange listener now clears the poll interval when hidden and restarts it when visible.
• CodeEditor.tsx: Early-return in tick() when hidden.
• MarkdownEditor.tsx: Early-return in tick() when hidden.
• BackgroundCommandOutputPanel.tsx: Early-return in interval callback when hidden.

Using document.visibilityState === 'hidden' instead of document.hidden for consistency and testability (jsdom does not sync hidden with visibilityState).

Validation

• tsc --noEmit — clean
• vitest run DispatchJobObserver.test.ts — 30/30 tests pass (2 new tests added: pause-when-hidden + resume-when-visible)

Fixes #1079