fix: use writable \$derived for search inputs, remove unnecessary \$state wrap

- Replace \$state + \$effect pattern with writable \$derived (Svelte 5.25+)
  for all searchValue instances across list pages — cleaner and lint-compliant
- Remove now-unused untrack imports from those files
- Drop \$state() wrapper around SvelteMap in device-mapping-dialog
  (SvelteMap is already reactive)

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

packages/frontend/src/routes/admin/videos/+page.svelte

@@ -1,5 +1,4 @@
 <script lang="ts">
-  import { untrack } from "svelte";
   import { goto, invalidateAll } from "$app/navigation";
   import { page } from "$app/state";
   import { SvelteURLSearchParams } from "svelte/reactivity";
@@ -18,8 +17,7 @@
   let deleteTarget: Video | null = $state(null);
   let deleteOpen = $state(false);
   let deleting = $state(false);
-  let searchValue = $state(untrack(() => data.search ?? ""));
-  $effect(() => { searchValue = data.search ?? ""; });
+  let searchValue = $derived(data.search ?? "");
   let searchTimeout: ReturnType<typeof setTimeout>;
 
   function debounceSearch(value: string) {