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/api/settings/+server.ts

@@ -5,7 +5,7 @@ import { log } from '$lib/server/logger';
 
 /**
  * GET /api/settings
- * Returns the current user's settings (auto_next, voice, speed, theme).
+ * Returns the current user's settings (auto_next, voice, speed, theme, locale, fontFamily, fontSize).
  * Returns defaults if no settings record exists yet.
  */
 export const GET: RequestHandler = async ({ locals }) => {
@@ -15,7 +15,10 @@ export const GET: RequestHandler = async ({ locals }) => {
 			autoNext: settings?.auto_next ?? false,
 			voice: settings?.voice ?? 'af_bella',
 			speed: settings?.speed ?? 1.0,
-			theme: settings?.theme ?? 'amber'
+			theme: settings?.theme ?? 'amber',
+			locale: settings?.locale ?? 'en',
+			fontFamily: settings?.font_family ?? 'system',
+			fontSize: settings?.font_size ?? 1.0
 		});
 	} catch (e) {
 		log.error('settings', 'GET failed', { err: String(e) });
@@ -25,7 +28,7 @@ export const GET: RequestHandler = async ({ locals }) => {
 
 /**
  * PUT /api/settings
- * Body: { autoNext: boolean, voice: string, speed: number, theme?: string }
+ * Body: { autoNext: boolean, voice: string, speed: number, theme?: string, locale?: string, fontFamily?: string, fontSize?: number }
  * Saves user preferences.
  */
 export const PUT: RequestHandler = async ({ request, locals }) => {
@@ -46,6 +49,24 @@ export const PUT: RequestHandler = async ({ request, locals }) => {
 		error(400, `Invalid theme — must be one of: ${validThemes.join(', ')}`);
 	}
 
+	// locale is optional — if provided it must be a known value
+	const validLocales = ['en', 'ru', 'id', 'pt-BR', 'fr'];
+	if (body.locale !== undefined && !validLocales.includes(body.locale)) {
+		error(400, `Invalid locale — must be one of: ${validLocales.join(', ')}`);
+	}
+
+	// fontFamily is optional — if provided it must be a known value
+	const validFontFamilies = ['system', 'serif', 'mono'];
+	if (body.fontFamily !== undefined && !validFontFamilies.includes(body.fontFamily)) {
+		error(400, `Invalid fontFamily — must be one of: ${validFontFamilies.join(', ')}`);
+	}
+
+	// fontSize is optional — if provided it must be one of the valid steps
+	const validFontSizes = [0.9, 1.0, 1.15, 1.3];
+	if (body.fontSize !== undefined && !validFontSizes.includes(body.fontSize)) {
+		error(400, `Invalid fontSize — must be one of: ${validFontSizes.join(', ')}`);
+	}
+
 	try {
 		await saveSettings(locals.sessionId, body, locals.user?.id);
 	} catch (e) {