css: moved + tweaked styles to common.css

- Add baseline input[type="checkbox"] and input[type="radio"] styling
  in common.css (accent-color, size, cursor, flex-shrink)
- Give select a solid background (var(--bg-primary)) and its own focus rule
- Remove now-redundant checkbox accent-color/size from
  .admin-checkbox-label (form.css) and .param-checkbox (admin.css)
- Simplify .search-filter-select (repertoire.css) to inherit common
  select defaults (border, background, arrow icon)
- Keep all layout-specific classes in form.css and admin.css intact
- Add baseline input[type="checkbox"] and input[type="radio"] styling
  in common.css (accent-color, size, cursor, flex-shrink)
- Give select its own rule block with same shape as text inputs
  (transparent background, same padding/border/radius/focus)
- Remove now-redundant checkbox accent-color/size from
  .admin-checkbox-label (form.css) and .param-checkbox (admin.css)
- Simplify .search-filter-select (repertoire.css) to inherit common
  select defaults
- Keep all layout-specific classes in form.css and admin.css intact
- Remove bottom-border/border-radius:0 overrides from .admin-form,
  .admin-inline-form, .param-form, and .param-grid inputs/selects
- Change required-field indicator from border-bottom-style to
  border-style: dashed to work with full-border approach
- Update param-grid aria-invalid from border-bottom-color to border-color
- All text inputs, selects, and textareas now inherit the full-border
  style from common.css (border, border-radius, padding, focus ring)
- .password-gate input[password]: remove redundant padding override
- .retry-email-form input[email]: remove redundant border/border-radius/
  padding/box-sizing, keep only font-size (larger) and width
- .tfe-access-request-form input/textarea: remove broken references to
  undefined vars (--border, --background, --accent), now inherit from
  common.css. Remove redundant focus rule.
- .fhb-name-input: strip redundant padding/border/radius/font-size/font
- .admin-inline-form input/select: strip redundant font-size
- .param-checkbox: remove font-size (inherits from body)
- .param-checkbox small: remove redundant color + font-size (common.css small already sets both)
- .param-note: remove font-size
- .param-account-status: remove font-size
- .param-smtp-test-row label: remove display:block + font-size (common.css label)
- .param-smtp-status: remove font-size
- .param-grid label: remove font-size
- Remove .param-form legend padding override (now inherits common.css legend)
- Remove .param-danger-zone legend padding override
- Remove .param-export-zone legend padding override
- Remove .param-fieldset-inline legend entirely (only rule was padding)
- Remove .licence-explanation legend entirely (all properties identical to common.css legend)
- All fieldsets now consistently use common.css fieldset padding
  (0 var(--space-m) var(--space-m) var(--space-m))
- The common.css fieldset has padding-top: 0, which leaves checkboxes
  and other content tight against the legend. Add var(--space-s) top
  padding so the first content row has proper spacing from the legend.

app/templates/admin/parametres.php

@@ -263,7 +263,7 @@
                 </div>
             </div>
 
-            <fieldset class="param-fieldset-inline">
+            <fieldset>
                 <legend>Expéditeur par défaut</legend>
                 <div class="param-grid">
                     <div>
@@ -292,13 +292,13 @@
         </form>
 
         <!-- Test d'envoi -->
-        <fieldset class="param-fieldset-inline param-smtp-test">
+        <fieldset class="param-smtp-test">
             <legend>Tester l'envoi d'un e-mail</legend>
             <p>Envoie un e-mail de test via le relay SMTP configuré ci-dessus.</p>
             <?php if (!$smtpConfigured): ?>
                 <p class="param-note">⚠ Configurez le relay SMTP avant de pouvoir tester l'envoi.</p>
             <?php else: ?>
-                <form method="post" action="actions/smtp-test.php" class="param-form param-smtp-test-form">
+                <form method="post" action="actions/smtp-test.php" class="param-form">
                     <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
                     <div class="param-smtp-test-row">
                         <div>
@@ -472,7 +472,7 @@
                 <input type="hidden" name="action" value="remove_credentials">
                 <input type="hidden" name="redirect" value="/admin/parametres.php">
                 <input type="hidden" name="current_password_remove" value="">
-                <button type="submit" class="param-btn-danger">Supprimer le mot de passe</button>
+                <button type="submit" class="btn btn--danger">Supprimer le mot de passe</button>
             </form>
         </fieldset>
         <?php endif; ?>