feat(messages): implement integrated chat with voice/calls and tidy root go module

Add Discord-like messaging APIs, websocket realtime, smart suggestions, password vault flows, semantic indexing integration, and new /app/messages UI.

Add typing indicators, advanced message search filters, voice notes, browser-local optional transcription, and WebRTC call signaling (offer/answer/ice/hangup).

Clean root go.mod via go mod tidy and remove stale root go.sum.

frontend/src/components/layout/Sidebar.tsx

@@ -18,6 +18,7 @@ import {
   IconBrandGithub,
   IconClock,
   IconCalendar,
+  IconMessageCircle,
   IconLogout,
   IconBuilding,
   IconPlus,
@@ -33,6 +34,7 @@ const navigation = [
   { name: 'Calendar', href: '/app/calendar', icon: IconCalendar },
   { name: 'Files', href: '/app/files', icon: IconFolder },
   { name: 'Notes', href: '/app/notes', icon: IconNotebook },
+  { name: 'Messages', href: '/app/messages', icon: IconMessageCircle },
   { name: 'YouTube', href: '/app/youtube', icon: IconVideo },
   { name: 'Members', href: '/app/members', icon: IconUsers },
   { name: 'Learning', href: '/app/learning-paths', icon: IconSchool },