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/LanguageSwitcher.svelte

@@ -12,7 +12,7 @@ const activeLocale = $derived(getLocale().toUpperCase());
 	<button
 		type="button"
 		onclick={() => setLocale(localeMap[locale])}
-		class="font-sans tracking-widest transition-colors
+		class="rounded px-1 font-sans tracking-widest transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-accent
                {activeLocale === locale
 			? inverted
 				? 'font-bold text-white'