fix(header): consistent icon styling, focus rings, and responsive breakpoints

- Normalize all header icon buttons to white/65 + white/10 hover bg
- Fix guest person icon (img tag needs brightness-0 invert, not text color)
- Add missing focus-visible rings to ThemeToggle and LanguageSwitcher
- Use focus-visible:rounded on nav links so active underline stays sharp
- Bump burger/nav breakpoint from sm→lg to prevent overflow on tablets

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

frontend/src/lib/components/NotificationBell.svelte

@@ -154,7 +154,7 @@ onDestroy(() => {
 			: m.notification_bell_label()}
 		aria-expanded={open}
 		aria-haspopup="true"
-		class="relative rounded-sm p-2 text-ink-2 transition-colors hover:text-ink focus:outline-none focus-visible:ring-2 focus-visible:ring-accent"
+		class="relative rounded-sm p-2 text-white/65 transition-colors hover:bg-white/10 hover:text-white focus:outline-none focus-visible:ring-2 focus-visible:ring-accent"
 	>
 		<!-- Bell SVG -->
 		<svg