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/capabilities/default.json

@@ -20,6 +20,7 @@
       "allow": [
         { "path": "**" }
       ]
-    }
+    },
+    "notification:default"
   ]
 }