fix(site/src/pages/AgentsPage/components/ChatsSidebar): hide kebab while archiving spinner shows (#28393)

Since #28326 made the row actions trigger (kebab) persistently visible
on the selected chat, it overlapped the archiving spinner: both render
right-aligned in the same 28px box (the kebab is `absolute inset-0` over
the timestamp/spinner slot), so archiving the selected chat produced a
garbled spinner-plus-dots glyph. The overlap could previously only occur
transiently on hover; the persistent kebab made it guaranteed on the
active row.

Render the kebab only when the row is not showing the archiving spinner,
restoring the intent of the existing `ArchivingShowsSpinnerOnly` story
(which only covers a non-selected row, so it never caught this).

<details>
<summary>Diagnosis and validation</summary>

**Diagnosis**

- The only right-side loader in a sidebar row is the `Spinner` rendered
by `ChatTreeNode` when `isArchiving && archivingChatId === chat.id`.
- #28326 added `isActiveChat && "opacity-100"` to the kebab trigger,
which shares the same `w-7` box as the spinner. The spinner branch never
suppressed the trigger, only the timestamp span had the hover/active
`hidden` classes.
- Result: on the selected chat, `EllipsisVerticalIcon` and the 8-leaf
`Spinner` render on top of each other for the entire archive mutation.

**Fix**

- `ChatTreeNode.tsx`: gate the `DropdownMenu` on `!isArchivingThisChat`
so the spinner and trigger are mutually exclusive. Row context-menu
actions remain available and are already disabled while archiving.

**Validation**

- `pnpm test:storybook
src/pages/AgentsPage/components/ChatsSidebar/ChatsSidebar.stories.tsx`:
53 passed
- `make pre-commit`: pass (lint/ts, lint/go, gen, fmt, build)

</details>

_Generated by Coder Agents on behalf of @tracyjohnsonux._

site/src/pages/AgentsPage/components/ChatsSidebar/tree/ChatTreeNode.tsx

@@ -346,7 +346,7 @@ export const ChatTreeNode: FC<ChatTreeNodeProps> = ({
 									aria-label="Shared chat"
 								/>
 							)}
-							{hasMenuActions && (
+							{hasMenuActions && !isArchivingThisChat && (
 								<DropdownMenu>
 									<DropdownMenuTrigger asChild>
 										<Button