shell: collapsible nav groups + mobile-friendly settings

- Reorganize sidenav into collapsible groups (Tenancy, Data,
  Integrations, Communications, Observability, AI & Search) with
  Overview/Settings pinned at top/bottom. Group open/close persists in
  localStorage; the group containing the active route auto-opens.
  Icon-only collapsed rail flattens to a single icon column. Sub-items
  inside groups drop their per-item icons and indent under the header.
- Fix mobile sheet scroll — the nav couldn't reach items past viewport
  height. SheetContent is now flex-col h-svh, header shrink-0, nav
  flex-1 min-h-0 overflow-y-auto.
- Settings page mobile fixes: section nav wraps instead of horizontal
  scroll, top padding clears the floating actions pill, LLM config
  card header and rows stack on narrow widths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/settings.tsx

@@ -171,10 +171,10 @@ export default function SettingsRoute() {
 
   return (
     <AppShell>
-      <div className="grid gap-6 md:grid-cols-[14rem_1fr]">
+      <div className="grid gap-6 pt-10 md:grid-cols-[14rem_1fr] md:pt-0">
         <nav
           aria-label="Settings sections"
-          className="flex flex-row gap-1 overflow-x-auto md:flex-col md:gap-0.5"
+          className="flex flex-row flex-wrap gap-1 md:flex-col md:flex-nowrap md:gap-0.5"
         >
           {sections.map((s) => {
             const Icon = s.icon