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>

server/routers/__init__.py

@@ -10,5 +10,13 @@ from .features import router as features_router
 from .agent import router as agent_router
 from .spec_creation import router as spec_creation_router
 from .filesystem import router as filesystem_router
+from .assistant_chat import router as assistant_chat_router
 
-__all__ = ["projects_router", "features_router", "agent_router", "spec_creation_router", "filesystem_router"]
+__all__ = [
+    "projects_router",
+    "features_router",
+    "agent_router",
+    "spec_creation_router",
+    "filesystem_router",
+    "assistant_chat_router",
+]