Replace HTMX+PHP file upload queues with client-side JS

Drops the session-backed HTMX incremental upload system in favour of a
single JS module that manages `File` objects client-side and injects
them into `FormData` on submit.

Key changes:

* `file-upload-queue.js`: client-side queues with validation, reorder
  (SortableJS), removal, dirty-state tracking, and fetch-based submit
  with manual redirect handling
* `fichiers-fragment.php`: empty queue containers for JS-managed queues;
  HTMX format switching still works with queue rehydration after swap;
  annexe uploads now support multiple files
* Form UI cleanup: moved existing files and cover preview into the
  `Fichiers` fieldset (edit mode); removed redundant queue labels while
  keeping labels for single-file inputs (`couverture`,
  `note d'intention`); added delete buttons for existing files
* `ThesisFileHandler.php`: added
  `handleTfeQueueFiles()`/`handleAnnexeQueueFiles()` reading from
  `$_FILES['queue_file']`; introduced `extractFilesSubArray()` for
  nested upload arrays; removed session-based queue handling
* `ThesisCreateController.php` &
  `ThesisEditController.php`: switched to extracted
  `['queue_file']` uploads
* `beforeunload-guard.js`: now also watches
  `window.__xamxamDirty`
* Deleted obsolete PHP upload/remove/reorder queue endpoints for
  `partage` and `admin`
* Cleaned up route dispatch in `partage/index.php`
* Misc form and styling updates in templates/CSS
* Added `docs/cms-migration-plan.html`

app/templates/partials/form/form.php

@@ -338,109 +338,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
         unset($_savedPost);
         ?>
 
-        <!-- Edit-only: existing files management -->
-        <div id="edit-existing-files-block">
-            <!-- Existing thesis files — with labels -->
-            <?php $thesisFilesList = array_values(
-                array_filter(
-                    $currentFiles,
-                    fn($f) => $f["file_type"] !== "cover",
-                ),
-            ); ?>
-            <?php if (!empty($thesisFilesList)): ?>
-            <div class="admin-form-group">
-                <label>Fichiers du TFE existants :</label>
-                <ul id="existing-files-sortable" class="admin-file-list">
-                    <?php foreach ($thesisFilesList as $f):
 
-                        $fExt = strtolower(
-                            pathinfo($f["file_path"] ?? "", PATHINFO_EXTENSION),
-                        );
-                        $fType = $f["file_type"] ?? "other";
-                        $fIcon = match (true) {
-                            $fType === "main" || $fExt === "pdf" => "📄",
-                            in_array($fExt, [
-                                "jpg",
-                                "jpeg",
-                                "png",
-                                "gif",
-                                "webp",
-                            ])
-                                => "🖼️",
-                            $fType === "video" ||
-                                in_array($fExt, ["mp4", "webm", "mov", "ogv"])
-                                => "🎬",
-                            $fType === "audio" ||
-                                in_array($fExt, [
-                                    "mp3",
-                                    "ogg",
-                                    "wav",
-                                    "flac",
-                                    "aac",
-                                    "m4a",
-                                ])
-                                => "🔊",
-                            $fType === "caption" || $fExt === "vtt" => "💬",
-                            $fType === "website" => "🌐",
-                            default => "📎",
-                        };
-                        $isExternalUrl =
-                            str_starts_with($f["file_path"] ?? "", "http://") ||
-                            str_starts_with($f["file_path"] ?? "", "https://");
-                        $fLinkHref = $isExternalUrl
-                            ? htmlspecialchars($f["file_path"])
-                            : "/media?path=" . urlencode($f["file_path"]);
-                        ?>
-                    <li class="admin-file-list-item" data-file-id="<?= (int) $f[
-                        "id"
-                    ] ?>">
-                        <input type="hidden" name="file_sort_order[]" value="<?= (int) $f[
-                            "id"
-                        ] ?>">
-
-                        <span class="admin-file-icon-col"><?= $fIcon ?></span>
-                        <span class="admin-file-info">
-                            <a href="<?= $fLinkHref ?>" target="_blank" rel="noopener" class="admin-file-name">
-                                <?= htmlspecialchars(
-                                    $f["file_name"] ??
-                                        basename($f["file_path"]),
-                                ) ?>
-                            </a>
-                            <span class="admin-file-meta-row">
-                                <span class="admin-file-type-badge"><?= htmlspecialchars(
-                                    $fType,
-                                ) ?></span>
-                                <?php if (
-                                    !empty($f["file_size"]) &&
-                                    $f["file_size"] > 0
-                                ): ?>
-                                <span class="admin-file-size"><?= number_format(
-                                    $f["file_size"] / 1024 / 1024,
-                                    2,
-                                ) ?> MB</span>
-                                <?php endif; ?>
-                            </span>
-                            <input type="text" name="file_label[<?= (int) $f[
-                                "id"
-                            ] ?>]"
-                                   value="<?= htmlspecialchars(
-                                       $f["display_label"] ?? "",
-                                   ) ?>"
-                                   placeholder="Légende / description (optionnel)"
-                                   class="admin-file-label-input">
-                        </span>
-                        <label class="admin-checkbox-label admin-file-delete">
-                            <input type="checkbox" name="delete_files[]" value="<?= (int) $f[
-                                "id"
-                            ] ?>"> Supprimer
-                        </label>
-                    </li>
-                    <?php
-                    endforeach; ?>
-                </ul>
-            </div>
-            <?php endif; ?>
-        </div><!-- #edit-existing-files-block -->
 <?php endif; ?>
 
         <!-- ═══════════════════ Métadonnées complémentaires ═══════════════════