feat(settings): add a back-to-hub link on the Layout child editors

The four Layout personalization editors (dashboard, insights, medications,
mood) are reached through the Layout & Personalization hub or a page-header
cog, and the left rail highlights the hub while the body shows the child.
The only return path was tapping the highlighted rail entry — no explicit
up-affordance, sharper on mobile where the rail collapses to a chip strip.
Add a shared back-to-hub link at the top of each child so the hub → child
→ hub loop reads clearly.

Also fix the stale doc-comment on the settings section route: the slug set
is 16 (12 nav-visible plus the four routable-but-hidden Layout children),
not eight.

src/app/settings/[section]/page.tsx

@@ -30,8 +30,10 @@ import {
 import { SettingsShell } from "@/components/settings/settings-shell";
 
 /**
- * Dynamic settings section route. Each of the eight `SETTINGS_SECTION_SLUGS`
- * is pre-rendered at build via `generateStaticParams()` so the URLs are
+ * Dynamic settings section route. Each of the `SETTINGS_SECTION_SLUGS`
+ * (16 today — 12 nav-visible sections plus the four routable-but-hidden
+ * Layout child editors) is pre-rendered at build via
+ * `generateStaticParams()` so the URLs are
  * statically known to Next.js, while the `dynamicParams = false` flag below
  * tells the router to 404 (instead of attempting on-demand rendering) for any
  * slug not in the list — which is exactly what `notFound()` would do at