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/src/types/reader.d.ts

@@ -209,6 +209,7 @@ interface ReaderSettings {
 
   double_page_spread: boolean;
   pdf_interaction_mode: "select" | "pan" | "text";
+  tap_zones_enabled: boolean;
   reading_direction: "ltr" | "rtl" | "vertical";
   reading_mode: "dark" | "light";