Add Linux D-Bus notification listener support

Refactor OS notification listener into platform backends:
- Windows: winsdk toast polling (unchanged behavior)
- Linux: dbus-next monitoring of org.freedesktop.Notifications
Add [notifications] optional dependency group in pyproject.toml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/pyproject.toml

@@ -60,6 +60,11 @@ dev = [
 camera = [
     "opencv-python-headless>=4.8.0",
 ]
+# OS notification capture
+notifications = [
+    "winsdk>=1.0.0b10; sys_platform == 'win32'",
+    "dbus-next>=0.2.3; sys_platform == 'linux'",
+]
 # High-performance screen capture engines (Windows only)
 perf = [
     "dxcam>=0.0.5; sys_platform == 'win32'",