Notifications: per-bridge enable + mute toggle

[tashda]

Why

With N bridges streaming logs, notification noise is multiplied. Users need per-bridge enable/mute and visual provenance.

What

• NotificationPreferences gains an optional per-bridge override map: [BridgeID: Set<NotificationCategory>]. If the bridge has no entry, fall back to global.
• InAppNotification carries bridgeID. The overlay displays a small bridge badge on each notification card.
• Add a "Mute notifications" toggle on each Saved Bridges row — silences bridge X without disconnecting it.

Files

• Shellbee/Core/Store/NotificationPreferences.swift — per-bridge override.
• Shellbee/Core/Store/AppStore+Notifications.swift — filter call respects per-bridge setting.
• Shellbee/Features/Notifications/InAppNotificationOverlay.swift — display badge.
• Shellbee/Features/Settings/SavedBridgesView.swift — mute toggle.

Verification

• Spam logs from bridge A via test center while bridge B is muted in saved-bridges settings; only bridge A notifications appear in the overlay.
• Mute bridge A and unmute bridge B; behavior reverses.

Phase

Phase 2 of multi-bridge support. Depends on #2.2.

[tashda]

Not addressed in the current multi-bridge batch (Phase 1 + Phases 2.1–2.5, 2.8, 2.9 shipped on dev). Per-bridge notification mute deferred — surface area and demand to be reassessed once multi-bridge ships.

[tashda]

Still relevant after the v1.6.0 multi-bridge release (#82). That PR explicitly deferred this, and NotificationPreferences remains a single global config with no per-bridge override map, and Saved Bridges rows have no mute toggle. With multiple bridges streaming logs, notification noise compounds — keeping this open.