feat: Add conversational AI assistant panel for project codebase Q&A

Implement a slide-in chat panel that allows users to ask questions about
their codebase using Claude Opus 4.5 with read-only access to project files.

Backend changes:
- Add SQLAlchemy models for conversation persistence (assistant_database.py)
- Create AssistantChatSession with read-only Claude SDK client
- Add WebSocket endpoint for real-time chat streaming
- Include read-only MCP tools: feature_get_stats, feature_get_next, etc.

Frontend changes:
- Add floating action button (bottom-right) to toggle panel
- Create slide-in panel component (400px width)
- Implement WebSocket hook with reconnection logic
- Add keyboard shortcut 'A' to toggle assistant

Key features:
- Read-only access: Only Read, Glob, Grep, WebFetch, WebSearch tools
- Persistent history: Conversations saved to SQLite per project
- Real-time streaming: Text chunks streamed as Claude generates response
- Tool visibility: Shows when assistant is using tools to explore code

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/lib/api.ts

@@ -14,6 +14,8 @@ import type {
   SetupStatus,
   DirectoryListResponse,
   PathValidationResponse,
+  AssistantConversation,
+  AssistantConversationDetail,
 } from './types'
 
 const API_BASE = '/api'
@@ -228,3 +230,40 @@ export async function validatePath(path: string): Promise<PathValidationResponse
     body: JSON.stringify({ path }),
   })
 }
+
+// ============================================================================
+// Assistant Chat API
+// ============================================================================
+
+export async function listAssistantConversations(
+  projectName: string
+): Promise<AssistantConversation[]> {
+  return fetchJSON(`/assistant/conversations/${encodeURIComponent(projectName)}`)
+}
+
+export async function getAssistantConversation(
+  projectName: string,
+  conversationId: number
+): Promise<AssistantConversationDetail> {
+  return fetchJSON(
+    `/assistant/conversations/${encodeURIComponent(projectName)}/${conversationId}`
+  )
+}
+
+export async function createAssistantConversation(
+  projectName: string
+): Promise<AssistantConversation> {
+  return fetchJSON(`/assistant/conversations/${encodeURIComponent(projectName)}`, {
+    method: 'POST',
+  })
+}
+
+export async function deleteAssistantConversation(
+  projectName: string,
+  conversationId: number
+): Promise<void> {
+  await fetchJSON(
+    `/assistant/conversations/${encodeURIComponent(projectName)}/${conversationId}`,
+    { method: 'DELETE' }
+  )
+}