[service-manager] Add option to hide notification content for privacy

[manusa]

Description

ElectronIM currently displays the full content of web notifications as native OS notifications.
For users working in public spaces or concerned about privacy, this can expose sensitive message content on their screen.

This feature adds an optional setting to hide notification content while still alerting users that a notification has been received.
When enabled, notifications will display a generic message instead of the actual notification content.

User Story

As a privacy-conscious user, I want to hide the content of notifications, so that sensitive information from my messages isn't visible on my screen when working in public spaces.

Acceptance Criteria

Settings UI

• Add a checkbox setting labeled "Hide notification content" in the Settings dialog under the Other tab
• Setting defaults to false (content visible)
• Setting persists across application restarts
• Setting applies immediately without requiring application restart

Notification Behavior

• When setting is enabled, all notifications display a generic message (e.g., "New message from [Service Name]")
• When setting is disabled, notifications display the original content from the web notification
• Notification icon and badge count remain unchanged regardless of setting
• Desktop notification sound/alert still triggers when setting is enabled
• Per-service notification settings (enabled/disabled) still work independently

Privacy Considerations

• No notification content is logged or stored when setting is enabled
• Setting works consistently across all platforms (Linux, Mac, Windows)

Testing Scenarios

• Enable setting, verify all notifications show generic content
• Disable setting, verify notifications show original content
• Test with multiple services simultaneously
• Verify setting persists after application restart
• Test notification clicking behavior with hidden content
• Verify Do Not Disturb mode still works with this setting
• Test on each platform (Linux, Mac, Windows)
• Verify notification badges/counts still work correctly