feat: add global in-app notification system

[JamesDAdams]

Summary

Adds a global in-app notification system to OpenFox:

• Header bell + unread badge — a notification button sits just left of the split-view button, with a green unread-count badge (capped at 99+) styled like the tasks badge.
• Notification center — a modal showing the full history (newest → oldest) with source, title, body, and date. Supports per-item delete (confirmed) and a "Clear all" action (confirmed).
• Toast notifications — a stacked toast appears top-right on receipt and auto-dismisses after 5 seconds. Multiple notifications stack.
• Notification settings — two master toggles in Settings → Notifications: "Popup notifications" (shows/hides the toast) and "Sound notifications" (plays a sound on receipt). Both persist.
• Plugin API — plugins can emit notifications via registry.notify({ title, body }), surfaced through the existing ProviderPluginRegistry.
• Persistence — notifications are stored in a new SQLite notifications table, so history survives page reloads and server restarts.
• Real-time sync — new/deleted/read/cleared notification events broadcast over WebSocket with fetch/reload parity (streamed and fetched data render identically).

AI-Enhanced Development

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• AI Models: DeepSeek V4 Flash

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Cache Impact

Does this PR affect anything cached — system prompts, tool definitions, skills, or other context?

• No