Refactor CSS architecture per css-methodology-spec.md

Split CSS into named layers: reset → colors → typography → base →
components → utilities. Each component has one unique root class in
its own file. No cross-component overrides.

New files:
- reset.css (modern-normalize base — matches project's prior reset)
- colors.css (all colour variables)
- typography.css (font faces, size/space scale, font-family vars)
- base.css (≤ 5 site-wide rules: layout, headings)
- utilities.css (sr-only, skip-link, reduced-motion)
- style.css (root @import file loading all layers)
- components/{links,focus,forms,tables,dialog,details,media,
  buttons,badges,toasts,pagination,header,search}.css

Existing files:
- variables.css → backward-compat wrapper (imports colors + typography)
- common.css → backward-compat wrapper (imports style.css)
- Page files (admin, public, form, tfe, apropos, repertoire, system,
  file-access) → removed redundant @import url(./variables.css)
- head.php → loads style.css instead of modern-normalize + common.css
- partage pages → load style.css

Fixes vs initial refactoring:
- reset.css: use modern-normalize base (not Tailwind Preflight) to
  avoid border/list/heading regressions from aggressive defaults
- components/search.css: restore !important flags on input styles
  (needed to override forms.css base input selectors)
- acces.php: add toast feedback on password copy button

Cleaned up duplicate status-badge/toast definitions from admin.css
(now live in components/badges.css and components/toast.css).

app/public/assets/css/components/toast.css

@@ -0,0 +1,65 @@
+/* ============================================================
+   TOAST — Bottom-center toast messages (CSS-only auto-fade).
+   Root class: .toast-container + .toast
+   ============================================================ */
+
+#toast-region {
+  position: fixed;
+  bottom: var(--space-l);
+  left: 50%;
+  transform: translateX(-50%);
+  z-index: 10000;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: var(--space-xs);
+  pointer-events: none;
+  width: max-content;
+  max-width: min(560px, calc(100vw - 2 * var(--space-l)));
+}
+
+.toast {
+  padding: var(--space-s) var(--space-m);
+  border-radius: var(--radius);
+  font-size: var(--step-0);
+  border-left: 4px solid;
+  box-shadow: 0 6px 24px rgba(0, 0, 0, 0.25);
+  pointer-events: auto;
+  /* enter then fade out — total visible ~6.35s */
+  animation: toast-enter 0.35s ease-out,
+             toast-exit  0.5s  ease-in  6s forwards;
+}
+
+.toast--error {
+  background: var(--bg-secondary);
+  border-color: var(--error);
+  color: var(--text-primary);
+}
+
+.toast--success {
+  background: var(--bg-secondary);
+  border-color: var(--success);
+  color: var(--text-primary);
+}
+
+.toast--warning {
+  background: var(--bg-secondary);
+  border-color: var(--warning);
+  color: var(--text-primary);
+  animation: toast-enter 0.35s ease-out;
+}
+
+.toast--warning a {
+  color: inherit;
+  text-decoration: underline;
+}
+
+@keyframes toast-enter {
+  from { opacity: 0; transform: translateY(12px); }
+  to   { opacity: 1; transform: translateY(0); }
+}
+
+@keyframes toast-exit {
+  from { opacity: 1; }
+  to   { opacity: 0; pointer-events: none; }
+}