Phase 3.2 docs + Phase 3.1 testing fixes

- Add phase3.2.md requirement: overlay UX, mitra activity log
- Add phase3.2-plan.md implementation plan
- Fix stale request validation: add GET /:sessionId/status endpoint
- Fix notification tap flow: setIncomingFromNotification + onChatRequestTapped
- IncomingRequestSheet shows stale message instead of auto-dismiss
- Home screen validates on resume, shows immediately on fresh WS

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/routes/public/mitra.chat.routes.js

@@ -1,6 +1,6 @@
 import { authenticate } from '../../plugins/auth.js'
 import { getMitraByFirebaseUid } from '../../services/mitra.service.js'
-import { acceptPairingRequest, declinePairingRequest } from '../../services/pairing.service.js'
+import { acceptPairingRequest, declinePairingRequest, getSessionStatus } from '../../services/pairing.service.js'
 import { getActiveSessionsByMitra, getActiveSessionsByMitraWithUnread, endSession, getMitraHistory } from '../../services/session.service.js'
 import { respondToExtension } from '../../services/extension.service.js'
 import { EndedBy } from '../../constants.js'
@@ -23,6 +23,15 @@ const resolveMitra = async (request, reply) => {
 }
 
 export const mitraChatRoutes = async (app) => {
+  // Check if a session is still pending acceptance (for notification validation)
+  app.get('/:sessionId/status', { preHandler: [authenticate, resolveMitra] }, async (request, reply) => {
+    const session = await getSessionStatus(request.params.sessionId)
+    if (!session) {
+      return reply.code(404).send({ success: false, error: { code: 'NOT_FOUND', message: 'Session not found' } })
+    }
+    return reply.send({ success: true, data: { status: session.status } })
+  })
+
   app.post('/:sessionId/accept', { preHandler: [authenticate, resolveMitra] }, async (request, reply) => {
     const session = await acceptPairingRequest(request.params.sessionId, request.mitra.id)
     return reply.send({ success: true, data: session })