fix: TFE and annexes files not saved, plus keyword validation and file preview CSS

- ThesisCreateController::submit() was missing call to handleAnnexeFiles
- ThesisEditController::save() was missing annexe upload handling
- handleAnnexeFiles now applies ALLOWED_MIME_TYPES/ALLOWED_EXTENSIONS validation
  (same restrictions as TFE files, formerly only size was checked)
- Use correct $_FILES key 'annexes' (matching the form input name)
- Relax keyword minimum: admin create/edit require 1+, student (partage) requires 3
- Add CSS styles for file preview items (.fp-item, .fp-thumb, .fp-icon,
  .fp-meta, .fp-name, .fp-size) so multi-file previews (annexes, etc.) wrap correctly
- Fix TFE file input accept attribute in fichiers-fragment.php to include
  video/audio/archive extensions

app/public/assets/css/form.css

@@ -1219,3 +1219,63 @@ a.recap-file-name:hover {
 .tag-search-item--create.tag-search-item--highlight {
   background: var(--accent-muted, rgba(149, 87, 181, 0.08));
 }
+
+/* ── File preview list (couverture, note d'intention, annexes) ───────────── */
+
+.file-preview-list {
+  list-style: none;
+  margin: var(--space-3xs) 0 0;
+  padding: 0;
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-3xs);
+}
+
+.fp-item {
+  display: flex;
+  align-items: center;
+  gap: var(--space-xs);
+  padding: var(--space-3xs) var(--space-xs);
+  background: var(--bg-secondary);
+  border: 1px solid var(--border-primary);
+  border-radius: var(--radius);
+  min-width: 0;
+  max-width: 100%;
+}
+
+.fp-thumb {
+  width: 48px;
+  height: 36px;
+  object-fit: cover;
+  border-radius: 3px;
+  flex-shrink: 0;
+}
+
+.fp-icon {
+  font-size: 1.3rem;
+  line-height: 1;
+  flex-shrink: 0;
+  width: 2rem;
+  text-align: center;
+}
+
+.fp-meta {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  flex: 1;
+  min-width: 0;
+}
+
+.fp-name {
+  font-size: var(--step--1);
+  font-weight: 500;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.fp-size {
+  font-size: var(--step--2);
+  color: var(--text-tertiary);
+}