Bootstrap 5 & remove jQuery (#661)

* build: move frontend assets to Bootstrap 5.3 and Tom Select

* feat: migrate templates and scripts to Bootstrap 5, dropping jQuery

Templates
- Rename Bootstrap 4 classes and data attributes (data-bs-*, ms-/me-/ps-/pe-,
  text-start/end, form-group -> mb-3, input-group wrappers removed, badges,
  custom controls -> form-check/form-select, media -> d-flex, btn-close, ...)
- Dark mode now uses Bootstrap 5.3 colour modes (data-bs-theme on <html>),
  set from <head> to avoid a flash of the light theme
- Remove the Bootstrap 4 /bootstrap/ demo page
- Contextual table classes (table-success etc.) still work as plain
  backgrounds for the rigboard key

JavaScript
- Remove jQuery: src.js/interaction.js and every inline template script are
  rewritten in plain JS on top of small helpers (helpers.js, bundled with
  Bootstrap into jpop.js so inline scripts can use them)
- Replace bootstrap-select/ajax-bootstrap-select with Tom Select. The remote
  search now sends the `q` parameter and reads `text`, which is what the
  API actually uses (the old picker sent `term` and read `label`)
- HTML fragments are fetched with an X-Requested-With header so views keep
  rendering their modal variants, and fetched scripts are re-executed
- Remove the Bootstrap 4 plugin script includes

* fix: stop rigboard header buttons stretching to full width under Bootstrap 5

* fix: restore dark-mode brand colours and contextual table colours under Bootstrap 5

* fix: brighten dark-mode links and make coloured card outlines visible

* fix: calendar layout and grid borders, navbar search button width, guard missing modal links

* fix: readable text in dark-mode Tom Select inputs and placeholders, restore form-control column widths

* build: split the Dockerfile into cached stages with prod and dev targets

* deploy: make compose.yml development-only with live reload, and document it

* fix: keep badge text colour for links inside badges

* fix: restore table-bordered grid lines in dark mode

* fix: include Bootstrap's progress styles and keep progress labels readable

pipeline/source_assets/scss/_custom-variables.scss

@@ -1,12 +1,14 @@
-$font-family-sans-serif:  "Open Sans", sans-serif;
+$font-family-sans-serif: "Open Sans", sans-serif;
 //Make it look less primary school
 $font-size-base: 0.85rem;
-$theme-colors: (
-  "success": #5cb85c,
-  "warning": #f0ad4e,
-  "danger": #d9534f,
-  "info": #5bc0de,
-  "primary": #3A52A2
-) !default;
 $enable-shadows: true;
-$alert-color-level: 10;
+$enable-rounded: false;
+
+$primary: #3A52A2;
+$success: #5cb85c;
+$warning: #f0ad4e;
+$danger: #d9534f;
+$info: #5bc0de;
+
+// Pre-computed: Sass serialises the derived (tinted) colour as rgb(...%), which breaks the url-encoded SVG
+$form-switch-focus-color: #9da9d1;