refactor(filters): mount-scope store bindings and fix effect-update loop

Replace the side-effect-on-import $effect.root in bindings with an explicit
startFilterBindings() started from an AppBindings provider in onMount, so the
filters/sort -> font-catalog bridge has a lifecycle tied to the app tree and a
returned cleanup. bindings now consumes getFontCatalog().

Fix the effect-update loop this surfaced: setGroups populated the reactive
groups array in place via `groups.length = 0; groups.push(...)`. push reads the
array's length signal, so the populating effect both read and wrote
groups.length each run and re-triggered itself forever
(effect_update_depth_exceeded). setGroups now reassigns the array (groups is
`let`), which does not read length.

Extract mapFilterMetadataToGroups to own the metadata -> group-config mapping,
including sorting a copy of options (the source is TanStack-cached data; an
in-place sort corrupts the cache and writes into the effect's read dependency).

src/app/App.svelte

@@ -16,12 +16,17 @@
  */
 import '$routes/router';
 import { Router } from 'sv-router';
-import { QueryProvider } from './providers';
+import {
+    AppBindingsProvider,
+    QueryProvider,
+} from './providers';
 import Layout from './ui/Layout.svelte';
 </script>
 
 <QueryProvider>
-    <Layout>
-        <Router />
-    </Layout>
+    <AppBindingsProvider>
+        <Layout>
+            <Router />
+        </Layout>
+    </AppBindingsProvider>
 </QueryProvider>

src/app/providers/AppBindings.svelte

@@ -0,0 +1,24 @@
+<!--
+    Component: AppBindings
+    Provider that starts app-wide store bindings (filters → sort → font catalog)
+    for its subtree. Mount-scoped so the bindings' lifetime tracks the app tree.
+-->
+<script lang="ts">
+import { startFilterBindings } from '$features/FilterAndSortFonts';
+import { onMount } from 'svelte';
+import type { Snippet } from 'svelte';
+
+interface Props {
+    /**
+     * Content snippet
+     */
+    children?: Snippet;
+}
+
+let { children }: Props = $props();
+
+// startFilterBindings returns its $effect.root cleanup; onMount runs it on unmount.
+onMount(() => startFilterBindings());
+</script>
+
+{@render children?.()}

src/app/providers/index.ts

@@ -1 +1,2 @@
+export { default as AppBindingsProvider } from './AppBindings.svelte';
 export { default as QueryProvider } from './QueryProvider.svelte';