Desktop Notification Fixes

Desktop notifications now display clean text, resolve user mentions to display names, and clicking them navigates reliably to the right conversation.

Improvements

User mentions in notifications now show the person's display name instead of the raw username format

Bug Fixes

Desktop notifications now show plain readable text instead of raw markdown symbols
Clicking a desktop notification now reliably opens and focuses the correct conversation
Notification previews for messages that couldn't be rendered now show "Sent a message" instead of incorrect fallback text
External links (such as the Stripe Connect setup button) now open correctly in the desktop app instead of silently failing

Desktop notifications have been overhauled to work reliably on the Tauri desktop app. Clicking a notification now correctly focuses the app and navigates to the relevant conversation, a flow that previously failed silently due to the Web Notification API not firing click events inside Tauri's webview.

Notification text is also cleaner: markdown formatting is stripped before display, and user mentions are resolved to display names so you see "@Alex" instead of "[[user:alex]]".