phsl / new-api

- Truncate long labels via pure CSS and always show full text in a Tooltip
- Ensure the right-side Tag is never truncated and remains fully visible
- Simplify implementation: remove overflow detection and ResizeObserver
- Use minimal markup with sbg-button/sbg-inner/sbg-label to enable shrinking
- Add global rules to allow `.semi-button-content` to shrink and ellipsize

Files:
- web/src/components/common/ui/SelectableButtonGroup.jsx
- web/src/index.css

No API changes; visuals improved and code complexity reduced.