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/providers/gemini/adapter.js

@@ -0,0 +1,186 @@
+/**
+ * Gemini provider adapter.
+ *
+ * Normalizes Gemini CLI session history into NormalizedMessage format.
+ * @module adapters/gemini
+ */
+
+import sessionManager from '../../sessionManager.js';
+import { getGeminiCliSessionMessages } from '../../projects.js';
+import { createNormalizedMessage, generateMessageId } from '../types.js';
+
+const PROVIDER = 'gemini';
+
+/**
+ * Normalize a realtime NDJSON event from Gemini CLI into NormalizedMessage(s).
+ * Handles: message (delta/final), tool_use, tool_result, result, error.
+ * @param {object} raw - A parsed NDJSON event
+ * @param {string} sessionId
+ * @returns {import('../types.js').NormalizedMessage[]}
+ */
+export function normalizeMessage(raw, sessionId) {
+  const ts = raw.timestamp || new Date().toISOString();
+  const baseId = raw.uuid || generateMessageId('gemini');
+
+  if (raw.type === 'message' && raw.role === 'assistant') {
+    const content = raw.content || '';
+    const msgs = [];
+    if (content) {
+      msgs.push(createNormalizedMessage({ id: baseId, sessionId, timestamp: ts, provider: PROVIDER, kind: 'stream_delta', content }));
+    }
+    // If not a delta, also send stream_end
+    if (raw.delta !== true) {
+      msgs.push(createNormalizedMessage({ sessionId, timestamp: ts, provider: PROVIDER, kind: 'stream_end' }));
+    }
+    return msgs;
+  }
+
+  if (raw.type === 'tool_use') {
+    return [createNormalizedMessage({
+      id: baseId, sessionId, timestamp: ts, provider: PROVIDER,
+      kind: 'tool_use', toolName: raw.tool_name, toolInput: raw.parameters || {},
+      toolId: raw.tool_id || baseId,
+    })];
+  }
+
+  if (raw.type === 'tool_result') {
+    return [createNormalizedMessage({
+      id: baseId, sessionId, timestamp: ts, provider: PROVIDER,
+      kind: 'tool_result', toolId: raw.tool_id || '',
+      content: raw.output === undefined ? '' : String(raw.output),
+      isError: raw.status === 'error',
+    })];
+  }
+
+  if (raw.type === 'result') {
+    const msgs = [createNormalizedMessage({ sessionId, timestamp: ts, provider: PROVIDER, kind: 'stream_end' })];
+    if (raw.stats?.total_tokens) {
+      msgs.push(createNormalizedMessage({
+        sessionId, timestamp: ts, provider: PROVIDER,
+        kind: 'status', text: 'Complete', tokens: raw.stats.total_tokens, canInterrupt: false,
+      }));
+    }
+    return msgs;
+  }
+
+  if (raw.type === 'error') {
+    return [createNormalizedMessage({
+      id: baseId, sessionId, timestamp: ts, provider: PROVIDER,
+      kind: 'error', content: raw.error || raw.message || 'Unknown Gemini streaming error',
+    })];
+  }
+
+  return [];
+}
+
+/**
+ * @type {import('../types.js').ProviderAdapter}
+ */
+export const geminiAdapter = {
+  normalizeMessage,
+  /**
+   * Fetch session history for Gemini.
+   * First tries in-memory session manager, then falls back to CLI sessions on disk.
+   */
+  async fetchHistory(sessionId, opts = {}) {
+    let rawMessages;
+    try {
+      rawMessages = sessionManager.getSessionMessages(sessionId);
+
+      // Fallback to Gemini CLI sessions on disk
+      if (rawMessages.length === 0) {
+        rawMessages = await getGeminiCliSessionMessages(sessionId);
+      }
+    } catch (error) {
+      console.warn(`[GeminiAdapter] Failed to load session ${sessionId}:`, error.message);
+      return { messages: [], total: 0, hasMore: false, offset: 0, limit: null };
+    }
+
+    const normalized = [];
+    for (let i = 0; i < rawMessages.length; i++) {
+      const raw = rawMessages[i];
+      const ts = raw.timestamp || new Date().toISOString();
+      const baseId = raw.uuid || generateMessageId('gemini');
+
+      // sessionManager format: { type: 'message', message: { role, content }, timestamp }
+      // CLI format: { role: 'user'|'gemini'|'assistant', content: string|array }
+      const role = raw.message?.role || raw.role;
+      const content = raw.message?.content || raw.content;
+
+      if (!role || !content) continue;
+
+      const normalizedRole = (role === 'user') ? 'user' : 'assistant';
+
+      if (Array.isArray(content)) {
+        for (let partIdx = 0; partIdx < content.length; partIdx++) {
+          const part = content[partIdx];
+          if (part.type === 'text' && part.text) {
+            normalized.push(createNormalizedMessage({
+              id: `${baseId}_${partIdx}`,
+              sessionId,
+              timestamp: ts,
+              provider: PROVIDER,
+              kind: 'text',
+              role: normalizedRole,
+              content: part.text,
+            }));
+          } else if (part.type === 'tool_use') {
+            normalized.push(createNormalizedMessage({
+              id: `${baseId}_${partIdx}`,
+              sessionId,
+              timestamp: ts,
+              provider: PROVIDER,
+              kind: 'tool_use',
+              toolName: part.name,
+              toolInput: part.input,
+              toolId: part.id || generateMessageId('gemini_tool'),
+            }));
+          } else if (part.type === 'tool_result') {
+            normalized.push(createNormalizedMessage({
+              id: `${baseId}_${partIdx}`,
+              sessionId,
+              timestamp: ts,
+              provider: PROVIDER,
+              kind: 'tool_result',
+              toolId: part.tool_use_id || '',
+              content: part.content === undefined ? '' : String(part.content),
+              isError: Boolean(part.is_error),
+            }));
+          }
+        }
+      } else if (typeof content === 'string' && content.trim()) {
+        normalized.push(createNormalizedMessage({
+          id: baseId,
+          sessionId,
+          timestamp: ts,
+          provider: PROVIDER,
+          kind: 'text',
+          role: normalizedRole,
+          content,
+        }));
+      }
+    }
+
+    // Attach tool results to tool_use messages
+    const toolResultMap = new Map();
+    for (const msg of normalized) {
+      if (msg.kind === 'tool_result' && msg.toolId) {
+        toolResultMap.set(msg.toolId, msg);
+      }
+    }
+    for (const msg of normalized) {
+      if (msg.kind === 'tool_use' && msg.toolId && toolResultMap.has(msg.toolId)) {
+        const tr = toolResultMap.get(msg.toolId);
+        msg.toolResult = { content: tr.content, isError: tr.isError };
+      }
+    }
+
+    return {
+      messages: normalized,
+      total: normalized.length,
+      hasMore: false,
+      offset: 0,
+      limit: null,
+    };
+  },
+};