feat(mobile): Figma-like hide-UI toggle — Cmd/Ctrl+\ hotkey + floating button, snapshot-exact chrome restore

Chrome visibility is now a runtime toggle on any device instead of being
device-wired: mobile defaults to canvas-only, desktop to full UI, and the
floating top-right button (or Figma's Cmd/Ctrl+\ chord — free in KiCad,
only bare \ is bound) flips between them live.

- chrome-visibility.ts: module-global store (default isMobileMode(),
  session-only) + pure hotkey matcher (rejects AltGr backslash + repeats)
- WasmTool: capture-phase hotkey (stopped before the wx layer), floating
  toggle pill (matches the comment FAB design), useLayoutEffect apply with
  sync first call + retry; overlays follow the toggle, capability-gated on
  the kicad_editor bundle's kicadSetChrome export
- boot.ts mobile opt now installs touch gestures only
- kicadSetChrome: frame-keyed hide-time snapshot so restore re-shows ONLY
  what hide took away (blanket Show(true) surfaced KiCad's default-hidden
  Search/Properties/Net-Inspector panes); toolbars-only fallback otherwise
- tests: chrome-toggle.spec.ts (desktop hide/restore + geometric
  restore-exactness ±3px), mobile toggle round-trip, 12 new unit tests,
  test:web:mobile npm script

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/package.json

@@ -13,6 +13,7 @@
     "setup:kicad": "./scripts/setup-kicad-wasm.sh",
     "test:web": "npm run setup:kicad && playwright test --config=playwright-web.config.ts --project=firefox",
     "test:web:headed": "npm run setup:kicad && playwright test --config=playwright-web.config.ts --project=chromium --headed",
+    "test:web:mobile": "npm run setup:kicad && playwright test --config=playwright-web.config.ts --project=mobile-chromium --headed",
     "test:kicad:firefox": "npm run setup:kicad && playwright test --config=playwright-kicad.config.ts --project=firefox",
     "test:kicad:chrome": "npm run setup:kicad && playwright test --config=playwright-kicad.config.ts --project=chromium --headed",
     "test:kicad": "npm run test:kicad:firefox",