feat(ui): language persistence, theme fix, font/size settings, header quick-access

- Fix language not persisting after refresh: save locale in user_settings,
  set PARAGLIDE_LOCALE cookie from DB preference on server load
- Fix theme change not applying: context setter was mismatched (setTheme vs current)
- Add font family (system/serif/mono) and text size (sm/md/lg/xl) user settings
  stored in DB and applied via CSS custom properties
- Add theme color dots and language picker to desktop header for quick access
- Footer locale switcher now saves preference to DB before switching
- Remove change password section (OAuth-only, no password login)
- Fix active sessions piling up: reuse existing session on re-login via OAuth
- Extend speed step cycle to include 2.5× and 3.0× (matching profile slider)
- Replace plain checkbox with modern toggle switch for auto-next setting
- Fix catalogue status labels (ongoing/completed) to use translation keys
- Add font family and text size translation keys to all 5 locale files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app.css

@@ -56,11 +56,18 @@ html {
 	color: var(--color-text);
 }
 
+/* ── Reading typography custom properties ──────────────────────────── */
+:root {
+	--reading-font: system-ui, -apple-system, sans-serif;
+	--reading-size: 1.05rem;
+}
+
 /* ── Chapter prose ─────────────────────────────────────────────────── */
 .prose-chapter {
 	max-width: 72ch;
 	line-height: 1.85;
-	font-size: 1.05rem;
+	font-family: var(--reading-font);
+	font-size: var(--reading-size);
 	color: var(--color-muted);
 }