feat: continuous reader mode with persisted preference

Add a vertical-scroll continuous mode to the reader alongside the
existing single-page mode. A segmented toggle in the reader top bar
switches between them; in continuous mode a gap selector
(None/Small/Medium/Large → 0/12/32/64px) controls the spacing
between stacked pages. Settings page mirrors the same controls.

Backend: new user_preferences table (one row per user, lazily
inserted, ON DELETE CASCADE) and GET/PATCH /api/v1/auth/me/preferences
gated by the existing CurrentUser extractor. Allowed values are
enforced both by API validation and table-level CHECK constraints.
Eight integration tests cover defaults, persistence, partial
updates, validation errors, auth, per-user isolation, and cascade.

Frontend: a new preferences store mirrors the theme-store pattern
with a localStorage shadow so anonymous browsers get a consistent
experience and logged-in users don't flash defaults while the
server response is in flight. Server values that the frontend
doesn't recognize (forward-compat) are ignored rather than poisoning
the UI; non-401 PATCH errors revert the optimistic local update;
logout clears the shadow so user A's settings don't follow user B
on a shared browser.

In continuous mode native scrolling handles Space/PageDown/arrows;
Home/End remain wired and call scrollIntoView() so jumping to chapter
bounds stays one keystroke. Single-page mode (chevrons, arrow-key
pagination, next-page preload) is unchanged.

Versions bumped 0.13.0 → 0.14.0 in lockstep.

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

backend/migrations/0008_user_preferences.sql

@@ -0,0 +1,8 @@
+CREATE TABLE user_preferences (
+    user_id          uuid PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    reader_mode      text NOT NULL DEFAULT 'single'
+        CHECK (reader_mode IN ('single', 'continuous')),
+    reader_page_gap  text NOT NULL DEFAULT 'none'
+        CHECK (reader_page_gap IN ('none', 'small', 'medium', 'large')),
+    updated_at       timestamptz NOT NULL DEFAULT now()
+);