refactor(app-updater): split AppUpdater.tsx into focused modules (#681)

Extract the GitHub release probe, download/relaunch state and the markdown
changelog renderer out of AppUpdater.tsx:

- utils/appUpdaterHelpers.ts   isNewer, fmtBytes, pickAsset, types
- hooks/useAppUpdater.ts        release probe + download/relaunch handlers
- components/appUpdater/Changelog.tsx

Pure code-move, no behaviour change. The AppShell call site is unchanged.

src/components/appUpdater/Changelog.tsx

@@ -0,0 +1,38 @@
+import React from 'react';
+
+// Minimal inline-markdown renderer (bold, italic, code)
+// IMPORTANT: regex must have NO nested capture groups — split() includes captured
+// groups in the result, and nested groups produce undefined entries that crash on .startsWith()
+function renderInline(text: string): React.ReactNode[] {
+  const parts = text.split(/(\*\*[^*]+\*\*|\*[^*]+\*|`[^`]+`)/g);
+  return parts.map((part, i) => {
+    if (!part) return null;
+    if (part.startsWith('**') && part.endsWith('**'))
+      return <strong key={i}>{part.slice(2, -2)}</strong>;
+    if (part.startsWith('*') && part.endsWith('*'))
+      return <em key={i}>{part.slice(1, -1)}</em>;
+    if (part.startsWith('`') && part.endsWith('`'))
+      return <code key={i} className="changelog-code">{part.slice(1, -1)}</code>;
+    return part;
+  });
+}
+
+/** Renders a GitHub release body as themed changelog markup. */
+export default function Changelog({ body }: { body: string }) {
+  return (
+    <>
+      {body.split('\n').map((line, i) => {
+        if (line.startsWith('### '))
+          return <div key={i} className="changelog-h3">{renderInline(line.slice(4))}</div>;
+        if (line.startsWith('#### '))
+          return <div key={i} className="changelog-h4">{renderInline(line.slice(5))}</div>;
+        if (line.startsWith('## '))
+          return null; // skip nested release headers in body
+        if (line.startsWith('- '))
+          return <div key={i} className="changelog-item">{renderInline(line.slice(2))}</div>;
+        if (line.trim() === '') return null;
+        return <div key={i} className="changelog-text">{renderInline(line)}</div>;
+      })}
+    </>
+  );
+}