feat(webui): support document attachments with ingress safeguards (#4771)

* feat: support document attachments in webui

* fix(webui): normalize document attachment MIME

* refactor(webui): move attachment policy out of channel

* fix(webui): reject oversized attachments before send

* fix(webui): align Portuguese attachment errors

* refactor(webui): separate ingress and transport limits

* fix(webui): reject malformed attachment payloads

webui/src/i18n/locales/en/common.json

@@ -909,7 +909,7 @@
         "edit": "Edit guidance",
         "drag": "Drag to reorder"
       },
-      "attachImage": "Attach image",
+      "attachImage": "Attach files",
       "imageMode": {
         "label": "Image Generation",
         "toggle": "Toggle image generation mode",
@@ -1034,12 +1034,17 @@
       "encoding": "Encoding…",
       "remove": "Remove attachment",
       "normalizedSizeHint": "{{orig}} → {{current}} (auto)",
+      "textTooLarge": "Message text is too large (max {{max}})",
       "imageRejected": {
         "unsupported_type": "Unsupported file type",
+        "empty_file": "Empty files cannot be attached",
+        "too_many_attachments": "Max {{max}} attachments per message",
         "too_many_images": "Max {{max}} images per message",
         "magic_mismatch": "File doesn't look like a real image",
         "decode_failed": "Couldn't decode this image",
-        "too_large": "Image is too large — try a smaller one",
+        "too_large": "File is too large — try a smaller one",
+        "total_too_large": "Attachments are too large together — remove some or use smaller files",
+        "transport_too_large": "This attachment would exceed the gateway transport limit",
         "io": "Couldn't read this file"
       },
       "workspace": {