Feature: Add governance notifications

[Mosas2000]

Summary: Add notifications for proposal creation, vote updates, execution, and cancellation events.

Why it matters: Users should not need to refresh the app constantly to know when governance state changes. Timely notifications make participation easier and reduce the chance that someone misses an important proposal window.

Scope:

• Define the events worth notifying users about.
• Provide a way to manage notification preferences.
• Keep notifications concise so they do not become noisy.

Acceptance notes:

• Important governance events trigger visible alerts.
• Users can opt in or out of specific categories.
• Duplicate notifications are minimized.