fix: correct github→gitea/codeberg refs and add privacy policy page

- Replace github.ref with gitea.ref in ci.yml (Forgejo uses gitea context)
- Fix README.md CI path from .github/ to .forgejo/
- Use codeberg icon instead of github icon in hugo.toml social link
- Add Privacy Policy page required for Google Play Store submission
- Add Privacy Policy link to website nav menu

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

website/hugo.toml

@@ -14,5 +14,10 @@ theme = 'PaperMod'
         Content = "The modern IMAP/SMTP email client for your desktop and mobile."
 
     [[params.socialIcons]]
-        name = "github"
+        name = "codeberg"
         url = "https://codeberg.org/guettli/sharedinbox"
+
+[[menu.main]]
+    name = "Privacy Policy"
+    url = "/privacy/"
+    weight = 10