feat(ui): apply metallic gradient to all buttons and surfaces
- Add gradient-button mixin for color-agnostic metallic button style
- Apply to all .btn-* variants via theme-color loop
- Reset hover lift inside .btn-group / .navbar to prevent button breakout
- Add gradient-surface-deep/shallow mixins for cards, modals, dropdowns
- Apply metallic surface gradients to .card, .card-header, .modal-content,
.dropdown-menu, .dropdown-item, .list-group-item
- Update gradient-accent to brighter stops (lighten 5% / darken 5%)
- Fix danger color contrast (#f85149 -> #d41111)
- Add --text CSS variable
- Clean up Dockerfile (remove unnecessary libc6/libstdc++6)
- Fix LactoseBaseUrl IP in docker-compose.yml
- Minor UI tweaks: badge spacing in AdminUsers, badge style in SettingRange