Clean up flash key legacy code and extract import.php inline styles

App::consumeFlash() had 18-line legacy fallback chains reading from seven old
session keys (error, admin_error, edit_error, form_error, success,
admin_success, edit_success) that were written by no code in the codebase.
All action handlers have used App::flash() -> _flash_error / _flash_success
since the App class was introduced. Removed the dead fallbacks; consumeFlash()
is now 4 lines.

admin/import.php was the last admin template with inline style= attributes.
Extracted four of them to named CSS classes in admin.css:
- admin-error-list   — error <ul> spacing (was style="margin:.5rem 0 0;padding-left:1.2rem")
- admin-file-hint    — <small> display + margin (was style="margin-top:.5rem")
- admin-import-results        — results panel margin (was style="margin-top:2rem")
- admin-import-results__title — <h2> typography (was multi-property inline style)

Closes the 'unify flash message keys' item in todo/02-php-components.md and
the import.php inline style item in todo/01-css-semantic-refactor.md.

public/admin/import.php

@@ -326,7 +326,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
     <?php if (!empty($errors)): ?>
     <div role="alert" data-type="error">
         <strong>⚠ Erreurs :</strong>
-        <ul style="margin:.5rem 0 0;padding-left:1.2rem;">
+        <ul class="admin-error-list">
             <?php foreach ($errors as $err): ?>
                 <li><?= htmlspecialchars($err) ?></li>
             <?php endforeach; ?>
@@ -345,7 +345,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
             <label>Fichier CSV :</label>
             <div class="admin-file-input">
                 <input type="file" id="csv_file" name="csv_file" accept=".csv" required>
-                <small style="margin-top:.5rem;">
+                <small class="admin-file-hint">
                     Colonnes attendues : Identifiant, Titre, Sous-titre, Auteur·ice(s), Contact, Promoteur·ice(s), Format, Année, AP, Orientation, Finalité, Mots-clés, Synopsis, Contexte, Remarques, Langue, Autorisation, License, taille, Points sur 20, lien BAIU<br>
                     — Deux premières lignes ignorées (en-tête) — Séparateur : virgule — Encodage : UTF-8
                 </small>
@@ -358,8 +358,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_FILES['csv_file'])) {
     </form>
 
     <?php if (!empty($importResults)): ?>
-    <div style="margin-top:2rem;">
-        <h2 style="font-size:1rem;font-weight:600;margin-bottom:.75rem;color:var(--text-secondary);text-transform:uppercase;letter-spacing:.06em;">Résultats de l'import</h2>
+    <div class="admin-import-results">
+        <h2 class="admin-import-results__title">Résultats de l'import</h2>
         <div class="info-message">
             <pre><?php foreach ($importResults as $r) echo htmlspecialchars($r) . "\n"; ?></pre>
         </div>

public/assets/css/admin.css

@@ -746,6 +746,34 @@
   gap: 1.5rem;
 }
 
+/* Error list inside role="alert" (import page) */
+.admin-error-list {
+  margin: 0.5rem 0 0;
+  padding-left: 1.2rem;
+}
+
+/* Hint text under the file input (import page) */
+.admin-file-hint {
+  display: block;
+  margin-top: 0.5rem;
+}
+
+/* Import results panel */
+.admin-import-results {
+  margin-top: 2rem;
+}
+
+.admin-import-results__title {
+  font-size: 1rem;
+  font-weight: 600;
+  margin-bottom: 0.75rem;
+  color: var(--text-secondary);
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  border: none;
+  padding: 0;
+}
+
 /* ── Jury fieldset ──────────────────────────────────────────────────────── */
 
 /* Nested lecteur·ices fieldset sits inside the outer jury fieldset: