docs(android): require reader settings parity with the web reader

The web reader's font roster (Literata default, plus seven self-hosted
variable fonts), typography controls, chrome/reading theme split,
fx brightness/contrast/invert stack, tap zones, and highlight palette
are the reference design for the Android reader - only the mobile
presentation differs. Document the mapping to the synced reader_settings
model so the app reuses it instead of inventing a parallel one.

docs/developer/android-app.md

@@ -96,6 +96,18 @@ Planned reading features:
 - Zoom and pan; aggressive preloading of adjacent pages
 - Webtoon / continuous vertical mode: post-v1
 
+### Reader settings parity with the web reader
+
+The web reader (`web/src/reader/`) is the reference implementation for reading ergonomics — its font selection, reading themes, and highlight system are considered well-designed; only its desktop-oriented presentation is being replaced on mobile. The Android reader should reuse the same settings model (stored in the `reader_settings` table and synced via the settings endpoint) rather than inventing a parallel one:
+
+- **Fonts**: the same roster of variable fonts, self-hosted under `/static/fonts/` — Literata (default), Crimson Pro, Source Serif 4, EB Garamond, Libertinus Serif, Noto Serif, Charis SIL, IBM Plex Serif (`FONT_MAP` in `web/src/reader/reader.ts`)
+- **Typography**: `font_size` (default 18), `line_height` (1.6), `margin_width`, `double_page_spread`
+- **Themes**: `chrome_theme` (default `tokyo-night`) for app chrome vs `reading_theme`/`reading_mode` for the page surface, plus the fx stack (`fx_brightness`, `fx_contrast`, `fx_invert`)
+- **Navigation**: `tap_zones_enabled` + `tap_zone_size`, `reading_direction`, `progress_mode`
+- **Highlights**: per-annotation color (default `#ffd54f`), matching the web palette
+
+Settings chosen on one device should follow the user everywhere — mobile changes write back through the same sync.
+
 ---
 
 ## 🍎 iOS Posture