feat(mental-health): gate the PHQ-9/GAD-7 check-in behind an opt-in module

The mental-wellbeing screener surface shipped built but unreachable: no nav
entry, no module key, ungated. Add a `mentalHealth` module key (opt-in,
default OFF — a depression/anxiety self-assessment is at least as sensitive
as mood) to the registry, the module-hub icon map, and the exhaustive module
maps. Give the check-in a nav home at /insights/mental-wellbeing, born-gated
on the module so it appears only once the account opts in, and redirect the
page home on a direct hit when the module is off.

Item answers stay encrypted and off the AI Coach / MCP regardless of the
toggle; this only controls whether the surface is visible and reachable.

src/components/layout/nav-model.ts

@@ -1,6 +1,7 @@
 import {
   Activity,
   Bell,
+  Brain,
   Droplets,
   FileScan,
   FlaskConical,
@@ -82,6 +83,19 @@ export const NAV_DESTINATIONS: ReadonlyArray<NavDestination> = [
     tourId: "nav-mood",
     requiresModule: "mood",
   },
+  // v1.25.0 — opt-in mental-health screeners (PHQ-9 / GAD-7), beside mood.
+  // Born-gated: `requiresModule: "mentalHealth"` reads the opt-in (default-off)
+  // key from the resolved module map, so the entry is absent until the account
+  // turns the module on from the Modules hub. The destination is the dedicated
+  // `/insights/mental-wellbeing` check-in surface (its own nav home — the
+  // active-detection helper defers `/insights` to it on that path).
+  {
+    href: "/insights/mental-wellbeing",
+    tKey: "nav.mentalWellbeing",
+    icon: Brain,
+    tourId: "nav-mental-wellbeing",
+    requiresModule: "mentalHealth",
+  },
   {
     href: "/cycle",
     tKey: "nav.cycle",

src/components/settings/modules-section.tsx

@@ -28,6 +28,7 @@ import {
   Activity,
   Award,
   Blocks,
+  Brain,
   CalendarHeart,
   Droplet,
   CloudSun,
@@ -80,6 +81,8 @@ const MODULE_ICONS: Record<ModuleKey, LucideIcon> = {
   mcp: Plug,
   // v1.25.0 (W-DOCS-IN) — inbound clinical documents (opt-in).
   inboundDocuments: FileScan,
+  // v1.25.0 — opt-in mental-health screeners (PHQ-9 / GAD-7).
+  mentalHealth: Brain,
 };
 
 export function ModulesSection() {