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/Dockerfile

@@ -18,7 +18,7 @@ RUN apt-get update && apt-get install -y \
 COPY pyproject.toml .
 COPY src/ ./src/
 COPY config/ ./config/
-RUN pip install --no-cache-dir .
+RUN pip install --no-cache-dir ".[notifications]"
 
 # Create directories for data and logs
 RUN mkdir -p /app/data /app/logs