fix(web): batch rapid stage clicks into one git command and one refresh

[MTVaught]

Problem

Every stage or unstage click in the diff panel became its own vcs.stagePaths call followed by a full preview regeneration (status plus two diffs). Staging many files quickly queued up dozens of git processes on the server and could overload it.

Fix

Clicks that arrive while a command is running are merged into the next one. The queue drains in batches, at most one stage and one unstage call per pass, and the preview refreshes once when the queue is empty. Per-file pending state, the optimistic hide, and the failure toast behave as before.

Follow-up to #100 and #102.

Built with Claude Fable 5.1 via Claude Code in T3 Code.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.6 KiB	+96 B (+0.7%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+4 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.5 KiB	+92 B (+1.4%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	57.0 KiB	+822 B (+1.4%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	+43 B (+0.3%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+2 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+41 B (+0.6%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.8 KiB	+44 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	8	9	+1 (+12.5%)	21	✅

Baseline: 1fd3553 · PR result: 3ce66d7 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.