refactor(ui): rename route /korrespondenz → /briefwechsel

Update all internal links (AppNav, CoCorrespondentsList, goto) to the
new URL. No redirect needed — no production URLs exist yet.

Refs: #179

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

frontend/src/routes/AppNav.svelte

@@ -60,9 +60,9 @@ function handleOverlayKeydown(event: KeyboardEvent) {
 		</a>
 
 		<a
-			href="/korrespondenz"
+			href="/briefwechsel"
 			class="my-2 inline-flex items-center px-3 font-sans text-xs font-bold tracking-widest uppercase transition-colors focus:outline-none focus-visible:rounded focus-visible:ring-2 focus-visible:ring-focus-ring
-                   {page.url.pathname.startsWith('/korrespondenz')
+                   {page.url.pathname.startsWith('/briefwechsel')
 				? 'border-b-2 border-accent text-white'
 				: 'text-white/70 hover:text-white'}"
 		>
@@ -161,9 +161,9 @@ function handleOverlayKeydown(event: KeyboardEvent) {
 				</a>
 
 				<a
-					href="/korrespondenz"
+					href="/briefwechsel"
 					class="block flex min-h-[44px] w-full items-center px-4 py-3 font-sans text-sm font-bold tracking-widest uppercase transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:ring-inset
-                           {page.url.pathname.startsWith('/korrespondenz')
+                           {page.url.pathname.startsWith('/briefwechsel')
 						? 'bg-accent-bg text-ink'
 						: 'text-ink-2 hover:bg-muted hover:text-ink'}"
 				>