luci-app-uci-git-backup: add LuCI interface

[mrangger]

Summary

• add a new luci-app-uci-git-backup application with settings and restore views for uci-git-backup
• include the LuCI menu entry, rpcd ACL, and German translations for the JS interface
• depend on the backend package proposed in uci-git-backup: add configuration backup package packages#29131

[github-actions]

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Commit b83a4be

• 🔶 Commit message lines should be <= 75 characters long
  Actual: line 2 is 135 characters long
  $\textsf{Add the modern LuCI JS views, menu entry, ACL, and translations for configu\color{red}{ring, testing, and restoring uci-git-backup from the web UI.}}$

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