Type-to-filter in file panels

Typing while the list is focused builds a filter query: a bar appears under
the column headers (search icon, query, live match count, Esc hint, clear
button) and the listing narrows to substring matches — '..' always stays.
Backspace edits the query, Escape clears it, cursor parks on the first match,
arrows/Enter keep working (the list never loses focus), and Space keeps its
select meaning until a query exists. Filter resets on directory change and
survives refresh; operations act only on visible (filtered) selection.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/panels.service.ts

@@ -24,10 +24,16 @@ export class PanelState {
   readonly sortAsc = signal(true);
   readonly cursor = signal(0);
   readonly selected = signal<ReadonlySet<string>>(new Set());
+  /** Type-to-filter query; entries not matching are hidden (".." always stays). */
+  readonly filter = signal('');
 
   /** Directories first, then the active sort column. ".." (parent) is pinned on top. */
   readonly sorted = computed(() => {
-    const list = [...this.entries()];
+    const query = this.filter().toLowerCase();
+    let list = [...this.entries()];
+    if (query) {
+      list = list.filter((e) => e.name.toLowerCase().includes(query));
+    }
     const key = this.sortKey();
     const dir = this.sortAsc() ? 1 : -1;
     list.sort((a, b) => {
@@ -69,6 +75,7 @@ export class PanelState {
       this.entries.set(res.entries);
       this.cursor.set(0);
       this.selected.set(new Set());
+      this.filter.set('');
     } finally {
       this.loading.set(false);
     }
@@ -110,6 +117,13 @@ export class PanelState {
     void this.load(`${this.cwd()}/..`);
   }
 
+  /** Update the filter and park the cursor on the first real match. */
+  setFilter(query: string): void {
+    this.filter.set(query);
+    const first = this.sorted().findIndex((e) => e.name !== '..');
+    this.cursor.set(first >= 0 ? first : 0);
+  }
+
   toggleSort(key: SortKey): void {
     if (this.sortKey() === key) {
       this.sortAsc.update((v) => !v);