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/language-search.php

@@ -24,7 +24,7 @@ $name           = $name           ?? 'language_autre';
 $label          = $label          ?? 'Autre(s) langue(s)';
 $placeholder    = $placeholder    ?? 'Rechercher une langue…';
 $hint           = $hint           ?? null;
-$hxPost         = $hxPost         ?? '/admin/fragments/language-search.php';
+$hxPost         = $hxPost         ?? '/admin/fragments/pill-search.php';
 $selectedLanguages = $selectedLanguages ?? [];
 $id             = $id             ?? $name;
 $maxLanguages   = $maxLanguages   ?? 10;
@@ -71,6 +71,7 @@ $langCount      = count($selectedLanguages);
                    hx-trigger="input changed delay:200ms, focus"
                    hx-target="#<?= htmlspecialchars($id) ?>-suggestions"
                    hx-swap="innerHTML"
+                   hx-vals='{"pill_type":"language"}'
                    hx-include="#<?= htmlspecialchars($id) ?>-pills">
         </div>