admin.css: rewrite from scratch using only variables.css tokens

The file had accumulated severe corruption in its lower half (garbled
selector text, variable names spliced into property values, orphaned
declarations, broken nesting) alongside hardcoded hex colours throughout.

Rewrote the entire file cleanly:
- Every colour is now a var() referencing a token defined in variables.css:
  --accent-primary/secondary/foreground, --accent-blue/green/yellow/red,
  --bg-secondary/tertiary, --border-primary, --text-primary/secondary/tertiary,
  --error, --warning, --success, --accent-muted.
- Zero raw hex values remain in admin.css.
- Removed the corrupted/dead CSS from the bottom half and reconstructed
  all selectors from what the templates actually use (audited via grep).
- Fixed structural issues: broken border shorthand, nested rules that
  were not valid CSS, orphaned declaration blocks.
- New/restored rules: .admin-maintenance-bar (was corrupted),
  .status-access variants (was corrupted), .admin-section-title--danger,
  .admin-danger-zone, .admin-account-status (all reconstructed cleanly).
- .admin-btn--warning and .admin-btn--danger now use var(--accent-yellow)
  and var(--accent-red) instead of hardcoded dark hex values.
- .admin-btn-remove hover now uses var(--error) instead of #e55.
- .admin-btn-unpublish now uses var(--bg-secondary)/var(--text-tertiary)
  instead of hardcoded grey hex values.
- select option background colours removed (browser chrome, not styleable
  cross-platform).

Templates: replace 4 inline var(--admin-text-muted) with var(--text-secondary)
in index.php, thanks.php, import.php.

public/admin/index.php

@@ -150,7 +150,7 @@ document.addEventListener('DOMContentLoaded', () => {
 
     <!-- Table -->
     <?php if (empty($theses)): ?>
-        <p style="color:var(--admin-text-muted);padding:1rem 0;">Aucun TFE trouvé.</p>
+        <p style="color:var(--text-secondary);padding:1rem 0;">Aucun TFE trouvé.</p>
     <?php else: ?>
     <table>
         <thead>
@@ -170,7 +170,7 @@ document.addEventListener('DOMContentLoaded', () => {
             <?php foreach ($theses as $thesis): ?>
             <tr>
                 <td><input type="checkbox" name="selected_theses[]" value="<?= $thesis['id'] ?>"></td>
-                <td style="color:var(--admin-text-muted);font-size:.8rem;"><?= htmlspecialchars($thesis['identifier'] ?? $thesis['id']) ?></td>
+                <td style="color:var(--text-secondary);font-size:.8rem;"><?= htmlspecialchars($thesis['identifier'] ?? $thesis['id']) ?></td>
                 <td>
                     <div class="thesis-title"><?= htmlspecialchars($thesis['title']) ?></div>
                     <?php if ($thesis['subtitle']): ?>