Instead of rendering error messages as `<small class="form-error">`
below inputs, errors now display as a circle-x icon inside the input
area. Hovering the icon shows the error text in a danger-styled tooltip.
- Wrap children in `.form-field-control` when error is present
- Use existing tooltip + icon components for the error indicator
- Add CSS for positioning, padding-right offset, and danger color overrides
- Update tests to match new structure
- Add pre-commit tmux server check instructions to AGENTS.md
When :href is provided, the button renders as <a> instead of <button>,
enabling navigation links with full button styling.
New :link variant renders as a minimal text link (underlined, accent
color, no background/padding) — works as both <button> and <a>.
CSS includes reset styles for a.btn (removes default link decoration)
and preserves underline for a.btn-link.
