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/routes/auth/[provider]/callback/+server.ts

@@ -25,7 +25,7 @@ import {
 	linkOAuthToUser
 } from '$lib/server/pocketbase';
 import { createAuthToken } from '../../../../hooks.server';
-import { createUserSession, mergeSessionProgress } from '$lib/server/pocketbase';
+import { createUserSession, touchUserSession, mergeSessionProgress } from '$lib/server/pocketbase';
 import { log } from '$lib/server/logger';
 
 type Provider = 'google' | 'github';
@@ -227,12 +227,21 @@ export const GET: RequestHandler = async ({ params, url, cookies, locals }) => {
 	);
 
 	// ── Create session + auth cookie ──────────────────────────────────────────
-	const authSessionId = randomBytes(16).toString('hex');
-	const userAgent = '' ; // not available in RequestHandler — omit
-	const ip = '';
-	createUserSession(user.id, authSessionId, userAgent, ip).catch((err) =>
-		log.warn('oauth', 'createUserSession failed (non-fatal)', { err: String(err) })
-	);
+	let authSessionId: string;
+
+	// Reuse existing session if the user is already logged in as the same user
+	if (locals.user?.id === user.id && locals.user?.authSessionId) {
+		authSessionId = locals.user.authSessionId;
+		// Just touch the existing session to update last_seen
+		touchUserSession(authSessionId).catch(() => {});
+	} else {
+		authSessionId = randomBytes(16).toString('hex');
+		const userAgent = ''; // not available in RequestHandler — omit
+		const ip = '';
+		createUserSession(user.id, authSessionId, userAgent, ip).catch((err) =>
+			log.warn('oauth', 'createUserSession failed (non-fatal)', { err: String(err) })
+		);
+	}
 
 	const token = createAuthToken(user.id, user.username, user.role ?? 'user', authSessionId);
 	cookies.set(AUTH_COOKIE, token, {