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/lib/markdown.ts

@@ -0,0 +1,57 @@
+import { marked } from "marked";
+import { markedHighlight } from "marked-highlight";
+import DOMPurify from "dompurify";
+import hljs from "highlight.js/lib/core";
+import "highlight.js/styles/atom-one-dark.min.css";
+
+// Register commonly used languages (keeps bundle small)
+import javascript from "highlight.js/lib/languages/javascript";
+import typescript from "highlight.js/lib/languages/typescript";
+import python from "highlight.js/lib/languages/python";
+import rust from "highlight.js/lib/languages/rust";
+import json from "highlight.js/lib/languages/json";
+import bash from "highlight.js/lib/languages/bash";
+import css from "highlight.js/lib/languages/css";
+import xml from "highlight.js/lib/languages/xml";
+import markdown from "highlight.js/lib/languages/markdown";
+import go from "highlight.js/lib/languages/go";
+import sql from "highlight.js/lib/languages/sql";
+import yaml from "highlight.js/lib/languages/yaml";
+
+hljs.registerLanguage("javascript", javascript);
+hljs.registerLanguage("js", javascript);
+hljs.registerLanguage("typescript", typescript);
+hljs.registerLanguage("ts", typescript);
+hljs.registerLanguage("python", python);
+hljs.registerLanguage("rust", rust);
+hljs.registerLanguage("json", json);
+hljs.registerLanguage("bash", bash);
+hljs.registerLanguage("sh", bash);
+hljs.registerLanguage("shell", bash);
+hljs.registerLanguage("css", css);
+hljs.registerLanguage("html", xml);
+hljs.registerLanguage("xml", xml);
+hljs.registerLanguage("markdown", markdown);
+hljs.registerLanguage("md", markdown);
+hljs.registerLanguage("go", go);
+hljs.registerLanguage("sql", sql);
+hljs.registerLanguage("yaml", yaml);
+hljs.registerLanguage("yml", yaml);
+
+// Configure marked with highlight.js via marked-highlight extension
+marked.use(
+  markedHighlight({
+    highlight(code: string, lang: string) {
+      if (lang && hljs.getLanguage(lang)) {
+        return hljs.highlight(code, { language: lang }).value;
+      }
+      return hljs.highlightAuto(code).value;
+    },
+  }),
+  { breaks: true },
+);
+
+export function renderMarkdown(md: string): string {
+  const html = marked(md) as string;
+  return DOMPurify.sanitize(html, { ADD_ATTR: ["id", "class"] });
+}