Add comprehensive soundpack manager with security-first design

- **Soundpack Manager**: Full-featured package discovery, download, and installation system
  - Repository management with HTTPS enforcement and validation
  - Directory listing support (auto-discovers .zip files) + soundpacknames.txt fallback
  - Secure download with size limits, timeout protection, and path sanitization
  - Zip bomb protection: file count, individual size, and total extraction limits
  - Audio file validation using magic numbers (not just extensions)
  - Accessible UI with keyboard navigation and screen reader optimization

- **Auto-refresh system**: Smart timeline updates respecting user activity
  - 300-second default interval + 10-second idle buffer
  - Keyboard activity tracking prevents interrupting active users
  - True new content detection using post IDs instead of counts
  - Preserves cursor position during background refreshes

- **Enhanced notifications**: Fixed spam issues and improved targeting
  - Timeline switching now silent (no notifications for actively viewed content)
  - Initial app load notifications disabled for 2 seconds
  - Generic "New content in timeline" messages instead of misleading post counts
  - Separate handling for auto-refresh vs manual refresh scenarios

- **Load more posts improvements**: Better positioning and user experience
  - New posts load below cursor position instead of above
  - Cursor automatically focuses on first new post for natural reading flow
  - Fixed widget hierarchy issues preventing activation

- **Accessibility enhancements**: Workarounds for screen reader quirks
  - Added list headers to fix Orca single-item list reading issues
  - Improved accessible names and descriptions throughout
  - Non-selectable header items with dynamic counts
  - Better error messages and status updates

- **Settings integration**: Corrected soundpack configuration management
  - Fixed inconsistent config keys (now uses [audio] sound_pack consistently)
  - Added soundpack manager to File menu (Ctrl+Shift+P)
  - Repository persistence and validation

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/notifications/notification_manager.py

@@ -106,4 +106,15 @@ class NotificationManager:
             title="Timeline updated",
             message=message,
             notification_type='timeline_updates'
+        )
+        
+    def notify_new_content(self, timeline_type: str = "timeline"):
+        """Show notification for new content without counting posts"""
+        if not self.is_enabled('timeline_updates'):
+            return
+            
+        self.show_notification(
+            title="Timeline updated",
+            message=f"New content in your {timeline_type}",
+            notification_type='timeline_updates'
         )