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/components/AssistantFAB.tsx

@@ -0,0 +1,35 @@
+/**
+ * Floating Action Button for toggling the Assistant panel
+ */
+
+import { MessageCircle, X } from 'lucide-react'
+
+interface AssistantFABProps {
+  onClick: () => void
+  isOpen: boolean
+}
+
+export function AssistantFAB({ onClick, isOpen }: AssistantFABProps) {
+  return (
+    <button
+      onClick={onClick}
+      className={`
+        fixed bottom-6 right-6 z-50
+        w-14 h-14
+        flex items-center justify-center
+        bg-[var(--color-neo-progress)] text-white
+        border-3 border-[var(--color-neo-border)]
+        rounded-full
+        shadow-neo-md
+        transition-all duration-200
+        hover:shadow-neo-lg hover:-translate-y-0.5
+        active:shadow-neo-sm active:translate-y-0.5
+        ${isOpen ? 'rotate-0' : ''}
+      `}
+      title={isOpen ? 'Close Assistant (Press A)' : 'Open Assistant (Press A)'}
+      aria-label={isOpen ? 'Close Assistant' : 'Open Assistant'}
+    >
+      {isOpen ? <X size={24} /> : <MessageCircle size={24} />}
+    </button>
+  )
+}