fix(editor): unify image upload flow for paste and button

Paste/drop and attachment button previously used separate upload paths.
The button uploaded first then called insertFile (which replaced the
current selection), while paste inserted a blob preview first. This
caused the second image to overwrite the first when both were used.

Now both paths share the same flow via uploadAndInsertFile():
blob preview with uploading animation → background upload → replace URL.

- Extract shared uploadAndInsertFile() function
- Replace insertFile ref method with uploadFile (inserts at doc end)
- Simplify FileUploadButton to onSelect(file) — no more onUpload/onInsert
- Wire onUploadFile in comment edit mode (was missing, upload was no-op)
- Unify image border-radius CSS for both editing and readonly modes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/features/modals/create-issue.tsx

@@ -419,8 +419,7 @@ export function CreateIssueModal({ onClose, data }: { onClose: () => void; data?
         {/* Footer */}
         <div className="flex items-center justify-between px-4 py-3 border-t shrink-0">
           <FileUploadButton
-            onUpload={handleUpload}
-            onInsert={(result, isImage) => descEditorRef.current?.insertFile(result.filename, result.link, isImage)}
+            onSelect={(file) => descEditorRef.current?.uploadFile(file)}
           />
           <Button size="sm" onClick={handleSubmit} disabled={!title.trim() || submitting}>
             {submitting ? "Creating..." : "Create Issue"}