perf: faster web delivery, smaller diffs, windowed conflicts

[erwin-wee]

Fixes from the performance audit (server mode on phones, large diffs, conflicts, startup, watcher).

Measured

	Before	After
Renderer JS	1,907 KB	919 KB (222 KB br)
CSS	126 KB	104 KB
Phone cold load (Fast 3G, CPU ×4)	12.7 s, 2.26 MB	2.8 s, 249 KB
Phone warm load	12.7 s, 2.06 MB	1.0 s, 10 KB
30k-line diff /invoke	4.17 MB	122 KB br
5,400-line conflict, mounted lines	5,400	66
File save → status refresh starts	162 ms	~6 ms

Diff compression was measured on a synthetic, highly repetitive file; expect roughly 5–10× on real code.

Changes

• Server static delivery: in-memory cache (revalidated by mtime so git-checkout rebuilds need no restart), Brotli/gzip negotiation, strong ETags + 304, immutable for hashed /assets/*, no-cache elsewhere. /gitgood-bridge.js and /invoke stay no-store. Large /invoke JSON compressed asynchronously.
• Renderer minification enabled (minify: 'esbuild'; electron-vite defaults it off).
• Diff payload: modified text diffs no longer send oldContent; reconstructOldLines rebuilds the old side from newContent + hunks. Covered by test/diff-reconstruct.test.ts against real git diff output (CRLF, no EOL at EOF, deletion at start, multi-hunk, rename).
• Conflict view windowed with useWindowedRows; lazy block highlighting extracted to lib/highlight.ts and shared with TextDiff (its cache now also resets when the reconstructed old side changes).
• Diff spinner only appears after 150 ms.
• Startup publishes settings/repos and opens the last repo without waiting for tool discovery or update state.
• Watcher fires on the first change after a quiet period; bursts still coalesce over 120 ms (1,000-file burst → 1 notification).
• Service worker (web mode only): cache-first hashed assets (max 12 entries), network-first navigations with an offline page. Never intercepts /invoke, the bridge, /version or /events.

Verification

• npm run typecheck, npm run build, npx vitest run: 90 files, 1,121 passed, 4 skipped.
• Isolated server + headless browser: headers/304/encodings via curl; phone load via CDP throttling; big-diff old-side highlighting and conflict windowing checked in the UI with screenshots (layout height unchanged).

Not done: the 40k-line diff still takes ~1.5 s, which is git diff itself; the spinner now covers it.