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/base.css

@@ -0,0 +1,44 @@
+/* ============================================================
+   BASE — Minimal site-wide rules. Keep this extremely small
+   (≤ 5 rules). Promote a pattern from a component to base
+   only once it's clearly universal.
+   ============================================================ */
+
+/* Full-height flex layout: header → main → (optional footer) */
+html, body {
+  height: 100%;
+  overflow: hidden;
+}
+
+body {
+  font-family: var(--font-body);
+  background: var(--bg-primary);
+  color: var(--text-primary);
+  background: linear-gradient(
+    180deg,
+    rgba(0, 0, 0, 0) 92%,
+    rgba(149, 87, 181, 1) 100%
+  );
+  display: flex;
+  flex-direction: column;
+}
+
+main {
+  flex: 1;
+  min-height: 0;
+  overflow-wrap: anywhere;
+}
+
+main * {
+  overflow-wrap: anywhere;
+  word-break: break-word;
+}
+
+/* Global heading scale — used by admin + public pages */
+:where(h1, h2, h3, h4, h5, h6) {
+  font-family: var(--font-display);
+  font-size: var(--step-2);
+  font-weight: 400;
+  margin: 0 0 var(--space-l) 0;
+  line-height: 1.15;
+}