add do not disturb mode with toggle and suppression

[romanstingler]

recording1.mp4

closes #629

This adds a Do Not Disturb (DnD) toggle that suppresses incoming toasts while keeping them in the notification list.

• DnD toggle in the notifications menu : a new bell-off button that flips DnD on/off. Critical/urgent notifications are never suppressed.
• DnD quick setting in the settings panel: a Bell/BellOff quick toggle, mirrored with the notification module state.
• Right-click to toggle the notifications module: right-clicking the bell icon toggles DnD directly from the bar.
• Icon: added BellOff to StaticIcon
• Config: remove_dnd_btn option under [settings] to hide the quick toggle.
• i18n + docs: settings-dnd string added in en-US/de-DE/fr-FR, plus docs for the new config flag and module behavior.

When DnD is active, non-critical toasts are suppressed (no pop-up), the bar shows the BellOff icon, and the quick setting is highlighted. Critical notifications still come through.

• DnD state is in-memory only (not persisted across restarts), happy to add persistence if you want it.
• The button only shows when the notifications module is enabled, and can be hidden entirely via remove_dnd_btn.