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/matrix.ts

@@ -221,12 +221,18 @@ export const uploadContent = async (
 
   const abortError = () =>
     matrixErrorFromUnknown(new DOMException('Upload cancelled', 'AbortError'));
+  // The card shows a plain sentence (describeUploadError); keep the raw error
+  // (status, errcode, URL) in the console for debugging.
+  const fail = (err: MatrixError) => {
+    if (err.data?.error !== 'Upload cancelled') console.warn('[upload] failed:', err);
+    onError(err);
+  };
 
   let lastError: MatrixError | undefined;
 
   for (let retryCount = 0; retryCount <= UPLOAD_MAX_RETRY_COUNT; retryCount += 1) {
     if (signal?.aborted) {
-      onError(abortError());
+      fail(abortError());
       return;
     }
     const uploadPromise = mx.uploadContent(file, {
@@ -246,13 +252,13 @@ export const uploadContent = async (
         return;
       }
       // Missing content_uri is not a transient failure — fail immediately.
-      onError(matrixErrorFromUploadResponse(data));
+      fail(matrixErrorFromUploadResponse(data));
       return;
     } catch (e: unknown) {
       lastError = matrixErrorFromUnknown(e);
 
       if (retryCount === UPLOAD_MAX_RETRY_COUNT || !isRetryableUploadError(e, !!signal?.aborted)) {
-        onError(lastError);
+        fail(lastError);
         return;
       }
 
@@ -265,14 +271,14 @@ export const uploadContent = async (
       await sleepForMs(waitMS);
       // Cancelled during the back-off — stop instead of resurrecting the upload.
       if (signal?.aborted) {
-        onError(abortError());
+        fail(abortError());
         return;
       }
     }
   }
 
   // Unreachable in practice, but keeps onError guaranteed if the loop exits.
-  if (lastError) onError(lastError);
+  if (lastError) fail(lastError);
 };
 
 export const matrixEventByRecency = (m1: MatrixEvent, m2: MatrixEvent) => m2.getTs() - m1.getTs();