Desktop Updates: surface release notes / What's new in update panel

## Goal

Add a `What's new` section to the `Desktop Updates` panel in Global Settings (and potentially the in-app update prompt), surfacing release notes / changelog entries for the available version.

## Context

- The new `AppUpdateAction.vue` component currently shows just version + state (`available` / `downloading` / `ready`).
- `@todesktop/runtime`'s `TDUpdateInfo` payload only exposes `releaseDate` and `version` — release notes are stripped from the upstream electron-updater `UpdateInfo`, so we cannot get them from the updater event itself.

## Proposed approaches

1. **GitHub Releases API** — fetch `https://api.github.com/repos/Comfy-Org/ComfyUI-Desktop-2.0-Beta/releases/tags/v{version}` and render the `body` as markdown. Pros: rich content, no extra infra. Cons: 60/hr unauthenticated rate limit per IP.
2. **Cloudflare-hosted changelog file** — publish a static `changelog.json` (or per-version markdown files) alongside the binary on our existing Cloudflare hosting, fetch on demand. Pros: no GitHub rate limit, can include richer / curated copy distinct from the auto-generated GitHub release body. Cons: extra publish step in the release pipeline.
3. **Bundled CHANGELOG.md** at build time — only useful for the *current* installed version, not the available one. Mostly a fallback.

## UX sketch

- Add a collapsible / expandable `What's new` section under the `Update X.Y.Z available` heading in the Desktop Updates settings panel.
- Lazy-fetch the body when the user expands it (or eagerly on transition to `available` if cheap).
- Render with a safe markdown component (sanitized — no raw HTML execution).
- Cache by version so toggling open/closed doesn't refetch.
- Title-bar pill stays terse — release notes only live in the settings panel (and possibly the system-modal restart prompt).

## Out of scope

- Showing release notes for *past* versions the user skipped.
- Diffing / aggregating multiple versions when the user is many releases behind (could be handled later by the changelog source).

## Related

Follow-up to e498e04 (title-bar+settings: unify app-update UI with download progress).