feat: unified message architecture with provider adapters and session store

- Add provider adapter layer (server/providers/) with registry pattern
    - Claude, Cursor, Codex, Gemini adapters normalize native formats to NormalizedMessage
    - Shared types.js defines ProviderAdapter interface and message kinds
    - Registry enables polymorphic provider lookup

  - Add unified REST endpoint: GET /api/sessions/:id/messages?provider=...
    - Replaces four provider-specific message endpoints with one
    - Delegates to provider adapters via registry

  - Add frontend session-keyed store (useSessionStore)
    - Per-session Map with serverMessages/realtimeMessages/merged
    - Dedup by ID, stale threshold for re-fetch, background session accumulation
    - No localStorage for messages — backend JSONL is source of truth

  - Add normalizedToChatMessages converter (useChatMessages)
    - Converts NormalizedMessage[] to existing ChatMessage[] UI format

  - Wire unified store into ChatInterface, useChatSessionState, useChatRealtimeHandlers
    - Session switch uses store cache for instant render
    - Background WebSocket messages routed to correct session slot

server/routes/gemini.js

@@ -1,39 +1,9 @@
 import express from 'express';
 import sessionManager from '../sessionManager.js';
 import { sessionNamesDb } from '../database/db.js';
-import { getGeminiCliSessionMessages } from '../projects.js';
 
 const router = express.Router();
 
-router.get('/sessions/:sessionId/messages', async (req, res) => {
-    try {
-        const { sessionId } = req.params;
-
-        if (!sessionId || typeof sessionId !== 'string' || !/^[a-zA-Z0-9_.-]{1,100}$/.test(sessionId)) {
-            return res.status(400).json({ success: false, error: 'Invalid session ID format' });
-        }
-
-        let messages = sessionManager.getSessionMessages(sessionId);
-
-        // Fallback to Gemini CLI sessions on disk
-        if (messages.length === 0) {
-            messages = await getGeminiCliSessionMessages(sessionId);
-        }
-
-        res.json({
-            success: true,
-            messages: messages,
-            total: messages.length,
-            hasMore: false,
-            offset: 0,
-            limit: messages.length
-        });
-    } catch (error) {
-        console.error('Error fetching Gemini session messages:', error);
-        res.status(500).json({ success: false, error: error.message });
-    }
-});
-
 router.delete('/sessions/:sessionId', async (req, res) => {
     try {
         const { sessionId } = req.params;