fix: migrate AI chat system from Firebase/Firestore to Postgres

Firebase was not configured so every chat request crashed with
'Firebase Admin credentials not configured'.

- chat-mode-resolver.ts: read project phase from fs_projects (Postgres)
- chat-context.ts: load project data from fs_projects instead of Firestore
- /api/ai/conversation: store/retrieve conversations in chat_conversations
  Postgres table (created automatically on first use)
- /api/ai/chat: replace all Firestore reads/writes with Postgres queries
- v_ai_chat/page.tsx: replace Firebase client auth with useSession from
  next-auth/react; remove Firestore listeners, use REST API for project data

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/server/chat-context.ts

@@ -5,7 +5,7 @@
  * building a compact context object for LLM consumption.
  */
 
-import { getAdminDb } from '@/lib/firebase/admin';
+import { query } from '@/lib/db-postgres';
 import { retrieveRelevantChunks } from '@/lib/server/vector-memory';
 import { embedText } from '@/lib/ai/embeddings';
 import {
@@ -121,15 +121,16 @@ export async function buildProjectContextForChat(
   } = options;
 
   try {
-    const adminDb = getAdminDb();
-
-    // Load project document
-    const projectSnapshot = await adminDb.collection('projects').doc(projectId).get();
-    if (!projectSnapshot.exists) {
+    // Load project from Postgres
+    const projectRows = await query<{ data: any }>(
+      `SELECT data FROM fs_projects WHERE id = $1 LIMIT 1`,
+      [projectId]
+    );
+    if (projectRows.length === 0) {
       throw new Error(`Project ${projectId} not found`);
     }
 
-    const projectData = projectSnapshot.data() ?? {};
+    const projectData = projectRows[0].data ?? {};
 
     // Load summaries in parallel
     const [knowledgeSummary, extractionSummary] = await Promise.all([