fix(web): /playlists/{id} page uses $app/state, not legacy $app/stores

svelte-check failed with 8 type errors: $app/stores' page.params.id
is typed `string | undefined`, and the page passed `id` raw to
helpers that need `string`. Two changes:

- Switch to `$app/state` (the project canonical for new pages — see
  routes/albums/[id]/+page.svelte). page.params.id is a direct
  property read, not a store subscription. Fall back to "" so id is
  always a string; SvelteKit won't actually render the page without
  a populated id, but the type-checker doesn't know that.
- Drop the now-unused `writable` import in the test file and update
  the mock to expose the new state-shape (object with .params, not a
  writable store).

The remaining a11y warnings (tabindex on role=menu/dialog, click
handlers without keyboard events, draggable div without role,
autofocus) are svelte-check warnings — they don't block the build.
Some are pre-existing in FlagPopover; others are tech debt from
slice 1's drag-and-drop and modal patterns. Address as a follow-up
polish task.

web/src/routes/playlists/[id]/+page.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { page } from '$app/stores';
+  import { page } from '$app/state';
   import { goto } from '$app/navigation';
   import { Pencil, Trash2 } from 'lucide-svelte';
   import { useQueryClient } from '@tanstack/svelte-query';
@@ -18,7 +18,11 @@
   import { playQueue } from '$lib/player/store.svelte';
   import type { TrackRef } from '$lib/api/types';
 
-  const id = $derived($page.params.id);
+  // SvelteKit types page.params.id as string | undefined (the route
+  // could in principle render with the slot empty); empty-string fallback
+  // keeps every downstream call typed and falls back to a 404 if the
+  // operator ever lands here without a real id.
+  const id = $derived(page.params.id ?? '');
   const queryClient = useQueryClient();
   const playlistStore = $derived(createPlaylistQuery(id));
   const playlistQuery = $derived($playlistStore);