Bootstrap's jQuery collapse (`data-toggle="collapse"` in Navigation.js, plus the
per-NavItem `data-target=".navbar-collapse.show"` auto-close) disappears when we
drop `alexwine:bootstrap-4` for `bootstrap@5`. Replace it with a React `useState`
that toggles `.show`, and close the mobile menu via an onClick on the nav <ul>.
Works on the current BS4 CSS (`.collapse.show` is pure CSS; jQuery only added the
height animation) and removes one of the two jQuery/BS4 JS blockers to the CSS
swap (the home carousel is the other). Also drops the dead sr-only toggler button
that targeted a non-existent id. No CSS changes.
Removes the largest batch of React-19-blocking warnings: the 0.31
components (Grid, FormGroup, ControlLabel, Navbar.Header/Brand, Checkbox,
SafeAnchor) all leaned on legacy context / defaultProps.
29 files converted: Grid->Container, FormGroup/ControlLabel/FormControl/
HelpBlock -> Form.Group/Label/Control/Text, Checkbox -> Form.Check (label
as prop), bsStyle->variant (default->secondary), bsSize->size,
pull-right->float-end. Custom Col.js now re-exports v2's Col; custom
NavItem.js rewritten self-contained (no SafeAnchor/createChainedFunction);
Navigation.js drops react-bootstrap Navbar (raw markup anyway).
CSS stays on Bootstrap 4 (alexwine:bootstrap-4) for now: the BS4->5 jump
is entangled with the jQuery carousel/swipe + navbar-collapse and is
tracked as separate debt in UPGRADE.md. Only .form-label needed a shim
(forms.scss). Browser-verified home carousel+navbar, signup form + terms
checkbox, login form; REST smoke byte-identical.
