feat: fix file deletion on save + trash policy + documents/ prefix + relink browser

1. note_intention: Delete old file only when a genuinely new upload arrives
   (32-char hex file_id), not when the FilePond pool preserves an existing
   file by sending its DB integer ID.  Previously the DB integer ID
   triggered $hasNewNote=true, which deleted the existing note_intention
   from disk+DB, then handleFilePondSingleFile couldn't re-process it
   because the regex requires a hex pattern.  Same fix applied to cover.

2. All file deletions now use deleteThesisFileToTrash() which renames
   files to tmp/_trash/ instead of unlinking.  The trash preserves
   original filenames prefixed with DB id for traceability.  Skips
   website URLs and PeerTube refs (no disk file).

3. Storage prefix changed from theses/ to documents/ to reflect that
   the folder holds all document types (determined by file_type in DB).
   MediaController visibility gate supports both prefixes for backward
   compat with existing files.

4. File browser + relink feature for orphaned files:
   - /admin/fragments/file-browser.php — HTMX tree browser for
     storage/documents/ and storage/theses/
   - /admin/actions/filepond/relink.php — POST endpoint that inserts
     a thesis_files row pointing to existing on-disk file
   - Per-pool "📂 Relier" buttons (edit mode only)
   - JS: XamxamOpenFileBrowser / XamxamRelinkFile with FilePond integration
   - CSS: .relink-modal dialog + .file-browser tree styles

app/templates/partials/form/fichiers-fragment.php

@@ -84,6 +84,14 @@ $websiteLabel = htmlspecialchars($_POST['website_label'] ?? '');
                    data-existing-files='<?= htmlspecialchars(json_encode($existingFilesJsonForCover ?? []), ENT_QUOTES) ?>'>
             <small>JPG, PNG ou WEBP. Format 4:3 recommandé. Max 20 MB.</small>
         </div>
+        <?php if ($editMode): ?>
+        <button type="button" class="btn btn--sm btn--ghost file-browser-trigger"
+                data-queue-type="cover"
+                data-thesis-id="<?= htmlspecialchars((string)($thesisId ?? $_GET['id'] ?? '')) ?>"
+                onclick="XamxamOpenFileBrowser(this)">
+            📂 Relier un fichier existant
+        </button>
+        <?php endif; ?>
     </div>
 
     <!-- ── 2. Note d'intention ── -->
@@ -98,6 +106,14 @@ $websiteLabel = htmlspecialchars($_POST['website_label'] ?? '');
                    <?= !$adminMode ? 'required' : '' ?>>
             <small>PDF uniquement. Max 100 MB. Si votre fichier est trop lourd, compressez-le avec <a href="https://www.bentopdf.com" target="_blank" rel="noopener">bentopdf.com</a>.</small>
         </div>
+        <?php if ($editMode): ?>
+        <button type="button" class="btn btn--sm btn--ghost file-browser-trigger"
+                data-queue-type="note_intention"
+                data-thesis-id="<?= htmlspecialchars((string)($thesisId ?? $_GET['id'] ?? '')) ?>"
+                onclick="XamxamOpenFileBrowser(this)">
+            📂 Relier un fichier existant
+        </button>
+        <?php endif; ?>
     </div>
 
     <!-- ── 3. TFE (all files: PDF, images, video, audio, VTT, archives) ── -->
@@ -122,6 +138,14 @@ $websiteLabel = htmlspecialchars($_POST['website_label'] ?? '');
                 <?php endif; ?>
             </small>
         </div>
+        <?php if ($editMode): ?>
+        <button type="button" class="btn btn--sm btn--ghost file-browser-trigger"
+                data-queue-type="tfe"
+                data-thesis-id="<?= htmlspecialchars((string)($thesisId ?? $_GET['id'] ?? '')) ?>"
+                onclick="XamxamOpenFileBrowser(this)">
+            📂 Relier un fichier existant
+        </button>
+        <?php endif; ?>
     </div>
 
     <!-- ── 4. Annexes ── -->
@@ -138,6 +162,14 @@ $websiteLabel = htmlspecialchars($_POST['website_label'] ?? '');
                        data-existing-files='<?= htmlspecialchars(json_encode($existingFilesJsonForAnnexe ?? []), ENT_QUOTES) ?>'>
                 <small class="admin-file-hint">PDF ou archives ZIP/TAR. Max 500 MB. Glissez pour réordonner.</small>
             </div>
+            <?php if ($editMode): ?>
+            <button type="button" class="btn btn--sm btn--ghost file-browser-trigger"
+                    data-queue-type="annexe"
+                    data-thesis-id="<?= htmlspecialchars((string)($thesisId ?? $_GET['id'] ?? '')) ?>"
+                    onclick="XamxamOpenFileBrowser(this)">
+                📂 Relier un fichier existant
+            </button>
+            <?php endif; ?>
         </div>
     </div>
 
@@ -154,3 +186,19 @@ $websiteLabel = htmlspecialchars($_POST['website_label'] ?? '');
 
 </fieldset><!-- /Fichiers -->
 </div><!-- #format-fichiers-block -->
+
+<!-- ═══════════════════ File Browser Modal (edit mode only) ═══════════════════ -->
+<?php if ($editMode): ?>
+<dialog id="relink-modal" class="relink-modal">
+    <div class="relink-modal-header">
+        <h3>Relier un fichier existant</h3>
+        <button type="button" class="btn btn--sm btn--ghost" onclick="document.getElementById('relink-modal').close()" aria-label="Fermer">✕</button>
+    </div>
+    <div id="relink-modal-body">
+        <p class="file-browser-loading">Chargement du navigateur de fichiers…</p>
+    </div>
+    <div class="relink-modal-footer">
+        <small>Seuls les fichiers déjà présents dans storage/documents/ ou storage/theses/ sont listés.</small>
+    </div>
+</dialog>
+<?php endif; ?>