feat: redesign UI for modern look and clean UX

- Refined dark theme with zinc palette and indigo accents
- Clean sidebar showing only project/session tree
- Moved pattern management to full-screen settings panel
- New session modal with project + pattern picker
- Modern chat with avatar icons, inline send button, Enter-to-send
- Welcome pane when no session is selected
- Added lucide-react for consistent iconography
- Custom scrollbar styling and auto-resize textarea
- Removed clutter: no sidebar pattern list, no verbose headers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -46,6 +46,7 @@
   },
   "dependencies": {
     "keytar": "^7.9.0",
+    "lucide-react": "^0.577.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4"
   }