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}"
 		>

web-player/src/lib/components/ui/overlay/AccountMenu.svelte

@@ -45,7 +45,7 @@
 
 	{#if open}
 		<GlassMenu class="absolute top-full right-0 z-(--z-menu) mt-2 overflow-hidden {WIDTH[width]}">
-			<MenuItem icon={User} label="Ver perfil" href="/u/{user.sub}" onclick={close} />
+			<MenuItem icon={User} label="Ver perfil" href="/user/{user.sub}" onclick={close} />
 			{@render extraItems?.({ close })}
 			{#if accountUrl}
 				<MenuItem
@@ -61,7 +61,7 @@
 				label={themeMode.current === 'dark' ? 'Modo blanco' : 'Modo oscuro'}
 				onclick={() => themeMode.toggle()}
 			/>
-			<form method="POST" action="/logout" data-sveltekit-reload>
+			<form method="POST" action="/auth/logout" data-sveltekit-reload>
 				<MenuItem icon={LogOut} label="Cerrar sesión" type="submit" />
 			</form>
 		</GlassMenu>