feat(settings): ship search filter that opens matching groups

The settingsSearch state was declared in Phase 9c with a comment
describing the intended behaviour but no <input> bound to it. With 21
SettingsGroup instances and a 2,261-line page, finding any specific
control required scrolling and opening every group. For the
neurodivergent audience this is exactly the cognitive-load failure
the brand commits to avoiding.

Added a sticky search input at the top of the page (paired with the
"Settings" title in a single sticky header that bg-bg-matches the
page so groups don't bleed under it). Each SettingsGroup now passes
through searchMatches against its title, description, and a curated
keyword string. Parent groups inherit their children's keywords so a
search for "GPU" expands AI & Processing and the AI Assistant inner
group together.

Updated SettingsGroup to push prop changes to the DOM via $effect:
the native <details> element's open attribute is mutated by user
clicks outside Svelte's reactive graph, so a parent prop change
needs to force-sync. User-driven toggles where the prop value didn't
change keep their local state because $effect only re-runs on diff.

Verified live: baseline shows 3 groups open (Transcription, Terms &
profiles, Capture & export), "GPU" opens AI & Processing + AI
Assistant only (other 19 closed), "ritual" opens Tasks & Rituals +
Rituals only, X-button clear restores baseline.

src/lib/components/SettingsGroup.svelte

@@ -21,6 +21,8 @@
   // Plain let (not $state) — the flag is mutated only by the toggle
   // handler and never has to be reactive in the template.
   let hasOpened = false;
+  let detailsEl: HTMLDetailsElement | null = $state(null);
+
   function handleToggle(event: Event) {
     const el = event.currentTarget as HTMLDetailsElement | null;
     if (el?.open && !hasOpened) {
@@ -28,9 +30,19 @@
       onopen?.();
     }
   }
+
+  // Push prop changes to the DOM. The native <details> open attribute
+  // is mutated by user interaction outside Svelte's reactive graph;
+  // when the parent later changes the open prop (for example because a
+  // search filter expanded a different group), $effect force-syncs.
+  // User-driven toggles where the prop value did not change keep their
+  // local state because $effect only re-runs on prop diff.
+  $effect(() => {
+    if (detailsEl && detailsEl.open !== open) detailsEl.open = open;
+  });
 </script>
 
-<details {open} ontoggle={handleToggle} class="settings-group border-t border-border-subtle">
+<details bind:this={detailsEl} {open} ontoggle={handleToggle} class="settings-group border-t border-border-subtle">
   <summary
     class="flex items-start gap-2 cursor-pointer list-none py-3 px-1 rounded hover:bg-hover focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
   >