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/form.css

@@ -1,7 +1,7 @@
 /* ============================================================
    FORM — TFE submission form
    Shared between admin add/edit pages and the student partage form.
-   Depends on variables.css (loaded via common.css).
+   Variables loaded via style.css (colors.css + typography.css).
    ============================================================ */
 
 /* ── Field rows & layout ────────────────────────────────────────────────── */
@@ -111,7 +111,7 @@
   margin-top: var(--space-2xs);
 }
 
-/* small base styles live in common.css */
+/* small base styles live in components/forms.css */
 
 /* ── Checkbox groups (languages, formats) ───────────────────────────────── */
 .admin-body fieldset.admin-checkbox-group,
@@ -147,7 +147,7 @@
 }
 
 /* ── Fieldsets & legends ────────────────────────────────────────────────── */
-/* Base fieldset/legend styles live in common.css */
+/* Base fieldset/legend styles live in components/forms.css */
 .admin-body fieldset,
 .student-body fieldset {
   margin: var(--space-2xs) 0 var(--space-s);
@@ -360,7 +360,7 @@
 /* ── Student-mode layout ────────────────────────────────────────────────── */
 
 /* Standalone page (no admin header/footer): restore scrollability that
-   common.css removes via overflow:hidden on html,body. */
+   base.css removes via overflow:hidden on html,body. */
 .student-body {
   display: flex;
   flex-direction: column;