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>

frontend/src/routes/+layout.svelte

@@ -2,6 +2,7 @@
     import { onMount, onDestroy } from 'svelte';
     import { goto } from '$app/navigation';
     import { logout } from '$lib/api/auth';
+    import { preferences } from '$lib/preferences.svelte';
     import { session } from '$lib/session.svelte';
     import { theme } from '$lib/theme.svelte';
     import Upload from '@lucide/svelte/icons/upload';
@@ -15,9 +16,17 @@
 
     onMount(() => {
         theme.init();
+        preferences.init();
         if (!session.loaded) session.refresh();
     });
 
+    // Pull fresh server preferences whenever the user changes (login,
+    // logout, account switch). The store's seq guard keeps the most recent
+    // response authoritative.
+    $effect(() => {
+        if (session.user) preferences.refresh();
+    });
+
     onDestroy(() => theme.destroy());
 
     async function handleLogout() {
@@ -26,6 +35,10 @@
             await logout();
         } finally {
             session.setUser(null);
+            // Don't let user A's reader preferences linger for the next
+            // person who uses this browser (or as guest state for the
+            // same user). Resets state + localStorage.
+            preferences.clearForLogout();
             loggingOut = false;
             goto('/login');
         }