Add syntax highlighting in code blocks and OS push notifications

Syntax highlighting: shared markdown renderer with highlight.js
(atom-one-dark theme), 12 language grammars registered (JS, TS,
Python, Rust, Go, Bash, JSON, YAML, SQL, CSS, HTML, Markdown).
Applied to both article reader and editor preview.

OS notifications: Tauri notification plugin for mentions, DMs, and
zaps. Per-type toggles in Settings with custom toggle switches.
Fires on new unread mentions/DMs; requests OS permission on first
enable. Notification utility at src/lib/notifications.ts.

src-tauri/src/lib.rs

@@ -144,6 +144,7 @@ pub fn run() {
         .plugin(tauri_plugin_http::init())
         .plugin(tauri_plugin_dialog::init())
         .plugin(tauri_plugin_fs::init())
+        .plugin(tauri_plugin_notification::init())
         .setup(|app| {
             // ── SQLite ───────────────────────────────────────────────────────
             let data_dir = app.path().app_data_dir()?;