feat: add in-app release notes viewer

Introduces a new feature to fetch, parse, and display app release notes
and update highlights directly within the UI.

- Backend: Adds the `release-notes` module to fetch GitHub releases
  and parse markdown highlight sections.
- Frontend: Adds the `WhatsNew` dialog and view, including custom
  markdown rendering and an image lightbox for update media.
- Tools: Adds validation scripts (`whats-new-lint`, `whats-new-preview`)
  to ensure release content parses correctly before publication.

server/src/config/config.ts

@@ -5,6 +5,8 @@ export const appConfig = registerAs('app', () => ({
   nodeEnv: process.env.NODE_ENV ?? 'development',
   appUrl: process.env.APP_URL ?? 'http://localhost:5173',
   version: process.env.APP_VERSION ?? 'Local build',
+  githubReleasesRepo: process.env.GITHUB_RELEASES_REPO?.trim() || 'bookorbit/bookorbit',
+  githubReleasesToken: process.env.GITHUB_RELEASES_TOKEN?.trim() || undefined,
   oidcAllowLocalIssuers: parseBooleanFlag(process.env.OIDC_ALLOW_LOCAL_ISSUERS, false),
   koboCloudscraperPython: process.env.KOBO_CLOUDSCRAPER_PYTHON?.trim() || undefined,
 }));