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/src/lib/components/TapZone.svelte

@@ -0,0 +1,70 @@
+<script lang="ts">
+    let {
+        onPrev,
+        onNext,
+        onToggle,
+        testid = 'tap-zone'
+    }: {
+        onPrev: () => void;
+        onNext: () => void;
+        onToggle: () => void;
+        testid?: string;
+    } = $props();
+</script>
+
+<div class="tap-zones" data-testid={testid}>
+    <button
+        type="button"
+        class="zone left"
+        onclick={onPrev}
+        aria-label="Previous page"
+        data-testid="{testid}-left"
+    ></button>
+    <button
+        type="button"
+        class="zone center"
+        onclick={onToggle}
+        aria-label="Toggle controls"
+        data-testid="{testid}-center"
+    ></button>
+    <button
+        type="button"
+        class="zone right"
+        onclick={onNext}
+        aria-label="Next page"
+        data-testid="{testid}-right"
+    ></button>
+</div>
+
+<style>
+    /* Invisible viewport overlay split into thirds. Mirrors the Mihon
+       / Tachiyomi convention: left-third = prev, center-third = toggle
+       chrome, right-third = next. `pointer-events: none` on the wrap
+       so unrelated clicks (chrome buttons, sheets) aren't trapped;
+       each zone re-enables pointer events locally. */
+    .tap-zones {
+        position: fixed;
+        inset: 0;
+        z-index: 5;
+        display: grid;
+        grid-template-columns: 1fr 1fr 1fr;
+        pointer-events: none;
+    }
+
+    .zone {
+        pointer-events: auto;
+        background: transparent;
+        border: 0;
+        padding: 0;
+        margin: 0;
+        cursor: pointer;
+        /* No visible style — these are interaction surfaces, not
+           controls. Focus-visible is honored so keyboard users get
+           a ring if they tab into one. */
+    }
+
+    .zone:focus-visible {
+        outline: 2px solid var(--focus-ring);
+        outline-offset: -4px;
+    }
+</style>