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.

package.json

@@ -18,12 +18,15 @@
     "@tauri-apps/plugin-dialog": "^2.6.0",
     "@tauri-apps/plugin-fs": "^2.4.5",
     "@tauri-apps/plugin-http": "^2.5.7",
+    "@tauri-apps/plugin-notification": "^2.3.3",
     "@tauri-apps/plugin-opener": "^2",
     "@tauri-apps/plugin-process": "^2.3.1",
     "@tauri-apps/plugin-updater": "^2.10.0",
     "@types/dompurify": "^3.0.5",
     "dompurify": "^3.3.2",
+    "highlight.js": "^11.11.1",
     "marked": "^17.0.4",
+    "marked-highlight": "^2.2.3",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-qr-code": "^2.0.18",