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/routes/+layout.svelte

@@ -46,7 +46,7 @@ const userInitials = $derived.by(() => {
 					<div class="flex items-center gap-3">
 						<!-- Language selector (desktop only — mobile lives in nav drawer) -->
 						<div
-							class="hidden items-center gap-1 pr-3 sm:flex [&_button]:px-1.5 [&_button]:py-1 [&_button]:text-xs"
+							class="hidden items-center gap-1 pr-3 lg:flex [&_button]:px-1.5 [&_button]:py-1 [&_button]:text-xs"
 						>
 							<LanguageSwitcher inverted />
 						</div>