feat: Add context menu for chat message interactions (#94)

Add right-click/long-press context menu to chat messages with event interactions:
- Reply to message
- Copy message text
- Open event detail (opens in new window)
- Copy event ID (nevent/naddr with relay hints)
- View raw JSON

Implements the same EventMenu pattern used in feed renderers, providing
consistent UX across the app. No visible UI elements added - menu appears
only on right-click or long-press (mobile).

Components added:
- src/components/ui/context-menu.tsx - Radix UI context menu primitives
- src/components/chat/ChatMessageContextMenu.tsx - Chat-specific context menu
- Integrated into MessageItem in ChatViewer.tsx

Dependencies:
- Added @radix-ui/react-context-menu

Co-authored-by: Claude <noreply@anthropic.com>

package.json

@@ -21,6 +21,7 @@
     "@radix-ui/react-avatar": "^1.1.11",
     "@radix-ui/react-checkbox": "^1.3.3",
     "@radix-ui/react-collapsible": "^1.1.12",
+    "@radix-ui/react-context-menu": "^2.2.16",
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-hover-card": "^1.1.15",