Consolidate auth routes under /auth and rename /u to /user

Moves the login screen from /login to /auth, logout from /logout to
/auth/logout (alongside the existing /auth/login and /auth/callback
endpoints), and the public profile route from /u/[id] to /user/[id].

AUTH_REDIRECT_URI and AUTH_POST_LOGOUT_URI already point at
/auth/callback and / respectively, so no Zitadel config change is
needed for the auth move.

web-player/src/lib/components/ui/media/ArtistLink.svelte

@@ -11,7 +11,7 @@
 {#if name}
 	{#if ownerUserId}
 		<a
-			href="/u/{ownerUserId}"
+			href="/user/{ownerUserId}"
 			onclick={(e) => e.stopPropagation()}
 			class="block max-w-full truncate hover:text-fg-2 hover:underline {klass}"
 		>