feat: Add notification email compression feature

Add intelligent notification email detection and compression:
- Detect notification emails from GitLab, GitHub, Jira, Confluence, Datadog, Renovate
- Extract structured summaries from notification emails
- Compress notifications into terminal-friendly markdown format
- Add configuration options for notification compression mode

Features:
- Rule-based detection using sender domains and subject patterns
- Type-specific extractors for each notification platform
- Configurable compression modes (summary, detailed, off)
- Integrated with ContentContainer for seamless display

Files added:
- src/mail/notification_detector.py: Notification type detection
- src/mail/notification_compressor.py: Content compression

Modified:
- src/mail/config.py: Add notification_compression_mode config
- src/mail/widgets/ContentContainer.py: Integrate compressor
- src/mail/app.py: Pass envelope data to display_content
- PROJECT_PLAN.md: Document new feature

src/mail/app.py

@@ -265,9 +265,17 @@ class EmailViewerApp(App):
         content_container = self.query_one(ContentContainer)
         folder = self.folder if self.folder else None
         account = self.current_account if self.current_account else None
-        content_container.display_content(message_id, folder=folder, account=account)
 
+        # Get envelope data for notification compression
         metadata = self.message_store.get_metadata(message_id)
+        envelope = None
+        if metadata:
+            envelope = self.message_store.envelopes.get(metadata["index"])
+
+        content_container.display_content(
+            message_id, folder=folder, account=account, envelope=envelope
+        )
+
         if metadata:
             message_date = metadata["date"]
             if self.current_message_index != metadata["index"]: