fix(frontend): keep #content portal target present on AI-session route (#9754)

The global fork modal (and other modals) portal into `#content`, but that
element only existed in AiChatLayout's `!disableAi` branch. On the AI-session
route `disableAi` is true, so the `{:else}` branch rendered without `#content`,
and opening the fork modal there threw "No element found matching css selector:
#content". Give the else-branch container the same `id` so the portal target is
always present in this layout.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/copilot/chat/AiChatLayout.svelte

@@ -101,6 +101,7 @@
 	</Splitpanes>
 {:else}
 	<div
+		id="content"
 		class={classNames(
 			'flex-1 min-h-0 flex flex-col',
 			noBorder || $userStore?.operator || isMobile ? '' : isCollapsed ? 'pl-12' : 'pl-40',