docs(modules): note the registry is the checklist for new pages

Review follow-up: the ordered MODULES array can't double as a compiler-
enforced exhaustive Page map, so document that adding a Page means adding it
here (an unregistered Page returns undefined from moduleDef/moduleLabelKey and
throws in NavSidebar). No behavior change. (Platform S6b.2)

frontend/src/lib/modules.ts

@@ -65,6 +65,10 @@ const SETTINGS_MODULE: ModuleDef = {
   icon: Settings,
 };
 
+// Registering a NEW Page means adding it to MODULES (or SETTINGS_MODULE) above — that's the single
+// place. If a Page is left unregistered, moduleDef()/moduleLabelKey[] return undefined and NavSidebar
+// throws on `.icon`; the coverage isn't compiler-enforced here because the ordered array can't also
+// be an exhaustive Page map, so treat the array as the checklist when the Page union grows.
 const BY_ID = Object.fromEntries(
   [...MODULES, SETTINGS_MODULE].map((m) => [m.id, m]),
 ) as Record<Page, ModuleDef>;