style(ui): migrate color scheme to macOS native design system

- Update primary blue from Linear style (#3498db) to macOS system blue (#0A84FF)
- Align gray scale with macOS dark mode palette (#1C1C1E, #2C2C2E, etc.)
- Adjust dark mode background to match macOS systemBackground (HSL 240 5% 12%)
- Refine scrollbar colors to use native macOS gray tones
- Remove transparent titleBarStyle for better stability

src-tauri/tauri.conf.json

@@ -19,8 +19,7 @@
         "minWidth": 800,
         "minHeight": 600,
         "resizable": true,
-        "fullscreen": false,
-        "titleBarStyle": "Transparent"
+        "fullscreen": false
       }
     ],
     "security": {