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/utils/appUpdaterHelpers.ts

@@ -0,0 +1,53 @@
+import { IS_LINUX, IS_MACOS, IS_WINDOWS } from './platform';
+
+export const SKIP_KEY = 'psysonic_skipped_update_version';
+
+// Semver comparison: returns true if `a` is newer than `b`
+export function isNewer(a: string, b: string): boolean {
+  const pa = a.replace(/^[^0-9]*/, '').split('.').map(Number);
+  const pb = b.replace(/^[^0-9]*/, '').split('.').map(Number);
+  for (let i = 0; i < 3; i++) {
+    if ((pa[i] ?? 0) > (pb[i] ?? 0)) return true;
+    if ((pa[i] ?? 0) < (pb[i] ?? 0)) return false;
+  }
+  return false;
+}
+
+export function fmtBytes(n: number): string {
+  if (n < 1024 * 1024) return `${(n / 1024).toFixed(0)} KB`;
+  return `${(n / 1024 / 1024).toFixed(1)} MB`;
+}
+
+export interface GithubAsset {
+  name: string;
+  browser_download_url: string;
+  size: number;
+}
+
+export interface ReleaseData {
+  version: string;
+  tag: string;
+  body: string;
+  assets: GithubAsset[];
+}
+
+export type DlState = 'idle' | 'downloading' | 'done' | 'error';
+
+export function pickAsset(assets: GithubAsset[]): GithubAsset | undefined {
+  if (IS_WINDOWS) {
+    return assets.find(a => a.name.endsWith('-setup.exe'))
+      ?? assets.find(a => a.name.endsWith('.exe'));
+  }
+  if (IS_MACOS) {
+    // Prefer Apple Silicon, fall back to Intel
+    return assets.find(a => a.name.endsWith('.dmg') && a.name.includes('aarch64'))
+      ?? assets.find(a => a.name.endsWith('.dmg'));
+  }
+  if (IS_LINUX) {
+    // AppImage > deb > rpm
+    return assets.find(a => a.name.endsWith('.AppImage'))
+      ?? assets.find(a => a.name.endsWith('.deb'))
+      ?? assets.find(a => a.name.endsWith('.rpm'));
+  }
+  return undefined;
+}