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/i18n/locales/de.json

@@ -2411,6 +2411,7 @@
       "channelBeta": "Beta",
       "beta": "BETA",
       "viewReleaseNotes": "Versionshinweise anzeigen",
+      "viewOnGithub": "PicPeak auf GitHub ansehen",
       "updateAvailableShort": "v{{version}} verfügbar",
       "upToDate": "Alles aktuell",
       "updateNow": "Jetzt aktualisieren",

frontend/src/i18n/locales/en.json

@@ -1987,6 +1987,7 @@
       "channelBeta": "Beta",
       "beta": "BETA",
       "viewReleaseNotes": "View Release Notes",
+      "viewOnGithub": "View PicPeak on GitHub",
       "updateAvailableShort": "v{{version}} available",
       "upToDate": "You're up to date",
       "updateNow": "Update Now",