feat(frontend): mobile reader with tap zones + settings sheet (0.59.0)

Phase 4 of the mobile redesign: the reader gets a mobile-native
interaction model — invisible tap zones for prev / next / toggle, a
chapter-jump bottom sheet, a reader settings sheet (mode / gap /
brightness), a fixed bottom page scrubber, a brightness overlay
driven by a CSS variable, and an idle-timer auto-hide for the chrome
after 3s of inactivity. Desktop keyboard shortcuts and chevrons are
preserved above 640px.

- New TapZone primitive: invisible 3-column grid that splits the
  viewport into prev / toggle / next thirds (Mihon convention). Wired
  to the existing `prev()` / `next()` / new `toggleChrome()` so
  chapter-edge behavior, page preload, and read-progress tracking
  all keep working. Has its own vitest coverage.
- matchMedia gates every mobile addition so the same DOM never
  carries two copies of the chapter selector or two sets of nav
  controls — the desktop <select>, mode toggle, and gap field stay
  inline above 640px; below 640px they swap to a chapter-jump
  button and a settings ⋯ button hosting the Sheets.
- Chapter jump Sheet lists every chapter with the current one
  highlighted; tapping a row navigates and dismisses.
- Reader settings Sheet uses the SegmentedControl primitive for mode
  and (continuous-only) page gap. Brightness lives next to them as a
  range slider 0.3..1; its value publishes `--reader-dim` as a CSS
  variable that drives the always-rendered fixed dim overlay
  (pointer-events: none so taps fall through). Brightness is
  client-side only in localStorage — the Preferences table doesn't
  carry it and Phase 4 deferred a backend migration.
- Idle auto-hide: 3s timer scoped to mobile + single mode, reuses
  the existing focus-mode CSS for the actual slide-off. The timer
  resets on every index/mode/viewport/fullscreen change via a tracked
  $effect.
- Bottom page scrubber: a styled <input type="range"> at the
  viewport foot, single + multi-page only, sliding off in focus mode
  alongside the chrome. Honors env(safe-area-inset-bottom).
- Reader joins the layout's `data-mobile-full-bleed` attribute so the
  hero/top reader-nav can sit at viewport top under main's cleared
  padding-top on mobile.
- Playwright spec covers tap left/right/center, the 3s auto-hide, the
  chapter-jump sheet, the settings sheet swapping to continuous, the
  brightness slider driving `--reader-dim`, and a desktop regression
  for the unchanged inline controls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.58.0",
+  "version": "0.59.0",
   "private": true,
   "type": "module",
   "scripts": {