filepond: fix crash 'can't access property main, n.status is undefined'

Fixes three root causes of FilePond errors on TFE upload forms:

1. server.process.onerror accessed .status on a string (XHR response
   text body) — now extracts the body safely.

2. server.load was a bare URL string with no error handling — converted
   to object with onload/onerror to prevent FilePond internal _write
   crash when load.php returns HTTP errors.

3. destroyFilePondsIn now aborts in-flight processing before pond.destroy()
   to prevent stale XHR callbacks firing on a torn-down FilePond instance.

Server-side: FilepondHandler now emits Content-Type: text/plain on all
responses (PHP defaults to text/html on die(), confusing FilePond's
response parser).

app/public/assets/js/app/file-upload-filepond.js

@@ -252,17 +252,21 @@
 				},
 				onload: (response) => {
 					var id = response.trim();
+					// Guard: if the server returned an error message disguised as 200,
+					// treat it as a processing error so FilePond doesn't treat it as a serverId.
+					if (id.length > 64 || /[<>\n\r]/.test(id)) {
+						console.error("[filepond] process onload | unexpected response | body=" + id.substring(0, 200));
+						throw new Error("Réponse serveur inattendue.");
+					}
 					console.log(`[filepond] process onload | serverId=${id}`);
 					return id; // file_id stored as serverId
 				},
 				onerror: (response) => {
-					console.error(
-						"[filepond] process onerror | status=" +
-							response.status +
-							" | body=" +
-							response,
-					);
-					return response;
+					// response is the raw XHR response text (string), not an XHR object.
+					// Log it and return a human-readable error message.
+					var body = typeof response === 'string' ? response : (response && response.body ? response.body : String(response || ''));
+					console.error("[filepond] process onerror | body=" + body);
+					return body || "Erreur lors du téléversement.";
 				},
 			},
 
@@ -274,11 +278,25 @@
 					console.log("[filepond] revert OK");
 				},
 				onerror: (r) => {
-					console.error(`[filepond] revert ERROR | body=${r}`);
+					var body = typeof r === 'string' ? r : (r && r.body ? r.body : '');
+					console.error(`[filepond] revert ERROR | body=${body || r}`);
 				},
 			},
 
-			load: `${base}/load.php?id=`,
+			load: {
+				url: `${base}/load.php?id=`,
+				method: "GET",
+				onload: (response) => {
+					// response is the blob from the server; pass through unchanged
+					return response;
+				},
+				onerror: (response) => {
+					var body = typeof response === 'string' ? response : (response && response.body ? response.body : String(response || ''));
+					console.error("[filepond] load onerror | body=" + body);
+					// Return a descriptive error — FilePond will fire an error event.
+					return body || "Fichier introuvable.";
+				},
+			},
 			// FilePond appends the source value (db_id) automatically
 
 			remove: (source, load, error) => {
@@ -500,6 +518,19 @@
 							}
 						}
 					}
+					// Abort any in-flight uploads before destroying to prevent
+					// FilePond internal crashes when XHR callbacks fire on a
+					// torn-down instance ("can't access property main").
+					var files = pond.getFiles();
+					for (var i = 0; i < files.length; i++) {
+						var f = files[i];
+						if (f.status === 4 || f.status === 2 || f.status === 3) {
+							// FileStatus: PROCESSING=4, PROCESSING_QUEUED=2, PROCESSING=4
+							// (FilePond 4.x internal: 4 = processing)
+							// Abort processing to avoid stale XHR callbacks
+							try { pond.removeFile(f); } catch (_abort) {}
+						}
+					}
 					pond.destroy();
 				} catch (_) {}
 			}