fix(upload): plain-language upload failure text instead of the raw MatrixError (#213)

The upload card printed the SDK's toString — 'MatrixError: [413] nope
(http://<hs>/_matrix/media/v3/upload?filename=…)'. describeUploadError() maps
the common cases to one sentence: 413/M_TOO_LARGE → 'This file is larger than
the server allows (limit N)' using m.upload.size when known, 429 → 'Slow down —
try again in a moment.', 401/403 → 'The server refused this upload: <server
text>', 5xx/transport after the retry loop → 'Couldn't reach the server. Check
your connection and retry.', other 4xx → the server's own sentence, URL
stripped. Both card renderers use it; the raw error is still console.warn-ed
by uploadContent for debugging. Unit-tested; verified headless with routed
413/403/503 responses.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/uploadError.ts

@@ -0,0 +1,37 @@
+import { MatrixError } from 'matrix-js-sdk';
+import { bytesToSize } from './common';
+
+/**
+ * [Gitea #213] One plain sentence for a failed upload. `MatrixError.message`
+ * is the SDK's toString — "MatrixError: [413] nope (http://hs/_matrix/media/…)"
+ * — which is what the card used to print. The raw error still goes to the
+ * console for debugging.
+ */
+export const describeUploadError = (error: MatrixError, limitBytes?: number): string => {
+  const status = error.httpStatus;
+  const errcode = error.errcode;
+  const serverText = (error.data as { error?: string } | undefined)?.error;
+
+  if (status === 413 || errcode === 'M_TOO_LARGE') {
+    const limit =
+      typeof limitBytes === 'number' && Number.isFinite(limitBytes)
+        ? ` (limit ${bytesToSize(limitBytes)})`
+        : '';
+    return `This file is larger than the server allows${limit}.`;
+  }
+  if (status === 429 || errcode === 'M_LIMIT_EXCEEDED') {
+    return 'Slow down — try again in a moment.';
+  }
+  if (status === 401 || status === 403 || errcode === 'M_FORBIDDEN') {
+    return serverText
+      ? `The server refused this upload: ${serverText}`
+      : 'The server refused this upload.';
+  }
+  if (typeof status !== 'number' || status === 408 || status >= 500) {
+    // Transport failure after the retry loop gave up, or a server-side error.
+    if (serverText === 'Upload cancelled') return 'Upload cancelled.';
+    return "Couldn't reach the server. Check your connection and retry.";
+  }
+  // Any other 4xx: the server's own sentence, never the URL-bearing toString.
+  return serverText ? serverText.replace(/\s*\(https?:\/\/[^)]*\)\s*$/, '') : 'Upload failed.';
+};