Extract last 3 inline styles from admin templates into CSS classes

admin/thanks.php:
- <div style="margin-top:1.5rem;display:flex;gap:.75rem;flex-wrap:wrap;"> → class="admin-action-bar"
- <p style="color:var(--text-secondary);"> → class="admin-muted"

admin/pages.php:
- Éditer button style="font-size:.8rem;padding:.3rem .75rem;" → class="admin-btn admin-btn--sm"

admin.css (Thesis info sections block):
- Added .admin-action-bar { margin-top:1.5rem; display:flex; gap:0.75rem; flex-wrap:wrap }
- Added .admin-muted { color: var(--text-secondary) }

The only remaining inline style in any admin PHP file is the dynamic
--disk-pct/--disk-color custom properties on the disk bar in system.php,
which carry PHP runtime values and cannot be moved to static CSS.

public/assets/css/admin.css

@@ -622,6 +622,17 @@
   font-size: 0.9rem;
 }
 
+.admin-action-bar {
+  margin-top: 1.5rem;
+  display: flex;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+}
+
+.admin-muted {
+  color: var(--text-secondary);
+}
+
 /* ── Section titles (account, etc.) ─────────────────────────────────────── */
 .admin-section-title {
   font-size: 1rem;