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/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() {