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/ThemeToggle.svelte

@@ -31,12 +31,12 @@ function toggle() {
 	onclick={toggle}
 	aria-label={theme === 'dark' ? 'light mode' : 'dark mode'}
 	title={theme === 'dark' ? 'light mode' : 'dark mode'}
-	class="rounded p-1.5 text-ink-2 transition-colors hover:bg-muted hover:text-ink"
+	class="rounded p-1.5 text-white/65 transition-colors hover:bg-white/10 hover:text-white focus:outline-none focus-visible:ring-2 focus-visible:ring-accent"
 >
 	{#if theme === 'dark'}
 		<!-- Sun icon — click to go light -->
 		<svg
-			class="h-4 w-4"
+			class="h-5 w-5"
 			viewBox="0 0 24 24"
 			fill="none"
 			stroke="currentColor"
@@ -52,7 +52,7 @@ function toggle() {
 	{:else}
 		<!-- Moon icon — click to go dark -->
 		<svg
-			class="h-4 w-4"
+			class="h-5 w-5"
 			viewBox="0 0 24 24"
 			fill="none"
 			stroke="currentColor"