feat(admin): GitHub repo button in the sidebar footer (#778)

Adds a subtle 'View PicPeak on GitHub' link in the admin sidebar footer
(next to the version/storage widgets), so admins can reach the repo —
star it, browse source, report an issue — from anywhere in the dashboard,
not just the setup screen.

- Centralizes the repo URL as `repoUrl` in utils/githubReleaseUrl.ts
  (githubReleaseUrl now derives from it) so the org URL lives in one place.
- target=_blank + rel=noopener noreferrer; EN + DE i18n
  (`admin.viewOnGithub`); dark-mode aware, matches the muted footer style.

frontend/src/utils/githubReleaseUrl.ts

@@ -10,5 +10,10 @@
  * notes for the running version (#566) and by the update-available
  * indicator to link to the upgrade target's notes.
  */
+/** Repository home on GitHub. Single source of truth for the org URL so
+ *  links (release notes, the admin "view on GitHub" button, #778) don't
+ *  each hardcode it. */
+export const repoUrl = 'https://github.com/PicPeak/picpeak';
+
 export const githubReleaseUrl = (version: string): string =>
-  `https://github.com/PicPeak/picpeak/releases/tag/v${version}`;
+  `${repoUrl}/releases/tag/v${version}`;