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/components/common/rich-text-editor.css

@@ -235,8 +235,8 @@
   display: inline;
 }
 
-/* Images in readonly mode */
-.rich-text-editor.readonly img {
+/* Images — shared styling for both editing and readonly */
+.rich-text-editor img {
   border-radius: var(--radius);
   margin: 0.5rem 0;
 }