fix(web): address self-review findings on #103 (master-safe defaults + hardening)

Make the branch genuinely master-mergeable and fix several review findings:

- Defaults are now prod-safe: CODEMAN_INSTANCE defaults to '' (→ ~/.codeman,
  -L codeman) and the web port back to 3000, so an existing install upgrades
  cleanly. Port also honors a new CODEMAN_PORT env var. Run the beta isolated
  alongside prod with scripts/run-beta.sh (CODEMAN_INSTANCE=beta + PORT 5000).
- .gitignore: anchor the root `public` symlink rule to `/public` (a bare
  `public` also swallowed src/web/public, silently un-staging new web assets);
  ignore the gesture wasm/model binaries explicitly instead.
- span-displays: add a macOS-only guard (400 elsewhere instead of spawning a
  bash that fails invisibly); extract resolveSpanUrl() for unit testing.
- server.ts: memoize asset-version stat() calls (~1s TTL) so each index render
  doesn't re-stat every script/link tag.
- styles.css: hide the multi-monitor button in solo (detached) windows.
- app.js: require two consecutive unanswered roll-calls before redocking, so a
  timer-throttled background popup isn't wrongly un-marked.
- index.html: make the "skip to terminal" link base-href-safe (onclick scroll)
  so it doesn't navigate to the dashboard from a /session/:id window.
- Tests: test/config/instance.test.ts, test/routes/system-span-displays.test.ts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -53,7 +53,17 @@ scripts/remotion/out/
 # Artifacts that should not be tracked
 test-results/
 tmp/
-public
+# Root `public` (a symlink to scripts/remotion/public — local artifact). ANCHORED
+# with a leading slash so it does NOT also match src/web/public (a bare `public`
+# would swallow the whole web UI source dir and silently un-stage any new asset
+# added there). No trailing slash so it still matches the symlink, not just dirs.
+/public
+
+# Opt-in gesture overlay runtime assets: large MediaPipe wasm + model (~27 MB)
+# fetched at build/install by scripts/fetch-gesture-assets.mjs, kept out of git.
+# (The gesture bundle itself, gesture-codeman.js, IS tracked.)
+src/web/public/gesture/wasm/
+src/web/public/gesture/*.task
 
 # Claude Code plan tracking
 plan.json