feat(reader): touch & mobile — tap zones, gesture engine, mobile sheets

Phase 2 of the reader redesign:

- Fixed-layout touch engine (foliate-js e9e61d8): pinch-zoom around
  the midpoint, two-finger pan, single-finger pan while zoomed,
  horizontal swipe page-turn at fit (RTL-aware via next()/prev()),
  and double-tap to zoom 2.5x / reset. Touch events forwarded from
  page iframes with converted coordinates; preventDefault only when
  the engine consumes the gesture, so PDF text selection and native
  taps stay intact. touch-action: none on the host and in comic/pdf
  page documents keeps the browser from fighting the engine.
- Tap zones (Kindle-style) for touch devices: tap the outer margins
  to page, center to toggle chrome. Size configurable (10-50%) via
  the revived tap_zone_size setting; toggle via new tap_zones_enabled
  (Behavior section of the settings drawer). Pointer-based + passive
  so drags/swipes/selection never trigger; attached both to the
  viewport and inside every page document (iframe events don't
  bubble); debounced 280ms so double-tap zoom doesn't also page; no
  zone actions while a fixed-layout page is zoomed.
- Drawers become full-width sheets on screens <= 640px.

web/static/input.css

@@ -859,6 +859,13 @@
     right: 0;
     border-left: 1px solid var(--border);
   }
+  /* Mobile: drawers become full-width sheets */
+  @media (max-width: 640px) {
+    .reader-drawer {
+      width: 100%;
+      max-width: 100%;
+    }
+  }
   .reader-drawer-header {
     display: flex;
     align-items: center;