fix(upload): surface payload-too-large errors, reject empty files, use toast not chat-log

Root cause of "uploads silently land as 0-byte __file": aiohttp default body
limit is 1 MiB; PDFs almost always exceed it, request.json() raises
HTTPRequestEntityTooLarge, the generic read_json() swallows it as bare
Exception, upload_handler then sees an empty dict and happily writes an
empty file with name='file', returning ok=true. Front-end thinks the
upload worked.

Bridge:
- Set web.Application(client_max_size=1MB) explicitly so the limit is
  visible at the call site (kept at 1MB intentionally for now; bump later
  if larger uploads are needed)
- upload_handler reads body itself instead of read_json so it can catch
  HTTPRequestEntityTooLarge and HTTPBadRequest and return a real error
  to the client instead of silently writing 0 bytes
- Reject blob == b'' as a second line of defence

Frontend:
- addFiles skips files with size === 0 (handles drag-folder ghost File
  and edge-case clipboard items) with a dedicated 'upload.empty' toast
- 4x showSystem(...) in addFiles -> showChanToast(title, detail, 'err').
  Upload failures should be transient toasts, not permanent system
  messages glued into the chat transcript.

frontends/desktop/static/app.js

@@ -269,7 +269,7 @@ const I18N = {
     'slash.unknown': '未知命令',
     'upload.hint': '上传文件：选择 / 拖拽 / 粘贴',
     'upload.button': '上传文件',
-    'upload.tooLarge': '文件过大或数量超限',
+    'upload.tooLarge': '文件过大或数量超限', 'upload.empty': '跳过空文件',
     'upload.failed': '上传失败',
     'upload.removeTitle': '移除',
     'upload.dropHint': '松开以上传文件',
@@ -362,7 +362,7 @@ const I18N = {
     'slash.unknown': 'Unknown command',
     'upload.hint': 'Upload file: pick / drag / paste',
     'upload.button': 'Upload file',
-    'upload.tooLarge': 'File too large or limit reached',
+    'upload.tooLarge': 'File too large or limit reached', 'upload.empty': 'Skipped empty file',
     'upload.failed': 'Upload failed',
     'upload.removeTitle': 'Remove',
     'upload.dropHint': 'Drop to upload files',
@@ -1572,14 +1572,17 @@ async function addFiles(fileList) {
   const files = Array.from(fileList || []);
   if (files.length === 0) return;
   let skipped = false;
+  let emptyHit = false;
   const accepted = [];
   for (const f of files) {
+    if (!f || f.size === 0) { emptyHit = true; continue; }
     if (f.size > MAX_UPLOAD_BYTES) { skipped = true; continue; }
     if (state.pendingFiles.length + accepted.length >= MAX_UPLOAD_FILES) { skipped = true; break; }
     accepted.push(f);
   }
+  if (emptyHit) showChanToast(t('upload.empty'), '', 'err');
   if (accepted.length === 0) {
-    if (skipped) showSystem(t('upload.tooLarge'));
+    if (skipped) showChanToast(t('upload.tooLarge'), '', 'err');
     return;
   }
   for (const f of accepted) {
@@ -1602,10 +1605,10 @@ async function addFiles(fileList) {
       insertPlaceholderInComposer(placeholderFor(entry));
       renderThumbStrip();
     } catch (e) {
-      showSystem(t('upload.failed') + ': ' + (e.message || e));
+      showChanToast(t('upload.failed'), e.message || String(e), 'err');
     }
   }
-  if (skipped) showSystem(t('upload.tooLarge'));
+  if (skipped) showChanToast(t('upload.tooLarge'), '', 'err');
 }
 
 if (uploadBtn && imgInput) uploadBtn.addEventListener('click', (e) => {

frontends/desktop_bridge.py

@@ -1081,7 +1081,14 @@ async def upload_handler(request):
     Body: {name: "<original filename>", dataUrl: "data:<mime>;base64,<...>"}
     Returns: {ok: true, path: "<abs path>"}
     """
-    data = await read_json(request)
+    try:
+        data = await request.json()
+        if not isinstance(data, dict):
+            data = {}
+    except web.HTTPRequestEntityTooLarge:
+        return json_ok({"ok": False, "error": "file too large for bridge body limit"})
+    except Exception as e:
+        return json_ok({"ok": False, "error": f"invalid request: {e}"})
     name = (data.get("name") or "file").strip().replace("/", "_").replace("\\", "_")
     data_url = data.get("dataUrl") or ""
     if "," in data_url:
@@ -1092,6 +1099,8 @@ async def upload_handler(request):
         blob = base64.b64decode(b64)
     except Exception as e:
         return json_ok({"ok": False, "error": f"decode failed: {e}"})
+    if not blob:
+        return json_ok({"ok": False, "error": "empty file"})
     safe_name = name or "file"
     fpath = _WEB_UPLOAD_DIR / f"{uuid.uuid4().hex[:12]}__{safe_name}"
     fpath.write_bytes(blob)
@@ -1216,7 +1225,7 @@ async def token_stats_handler(request):
 
 
 def create_app():
-    app = web.Application(middlewares=[cors_middleware])
+    app = web.Application(middlewares=[cors_middleware], client_max_size=1 * 1024 * 1024)
     app.router.add_get("/ws", ws_handler)
     app.router.add_get("/status", status_handler)
     app.router.add_get("/config", get_config_handler)