fix: ensure checkpoint restore popover works when portal container is not immediately available

[roomote]

Description

This PR attempts to address Issue #8563 where users reported that clicking on checkpoints does nothing. The issue was caused by the portal container element not being available when the CheckpointMenu component tried to render the popover.

Problem

• Clicking on checkpoints had no effect - the restore menu popover was not appearing
• The portal container element ('roo-portal') might not be immediately available in the DOM when components mount

Solution

1. Enhanced useRooPortal hook:

   • Replaced useMount with useEffect for better lifecycle management
   • Added MutationObserver to detect when the portal element is dynamically added to the DOM
   • Properly cleans up the observer on unmount to prevent memory leaks

2. Fixed CheckpointMenu component:

   • Added fallback to undefined when portal container is null
   • Ensures PopoverContent receives the correct prop type

Testing

• ✅ All existing tests pass (CheckpointSaved.spec.tsx)
• ✅ Components using useRooPortal (ModeSelector, ApiConfigSelector) remain unaffected
• ✅ Type checking passes
• ✅ Linting passes

Related Issues

Fixes #8563

Feedback and guidance are welcome!

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Important

Fixes checkpoint restore popover issue by enhancing useRooPortal with MutationObserver and handling null containers in CheckpointMenu.

• Behavior:
  • Fixes issue [BUG] Checkpoint exists but clicking does nothing #8563 where checkpoint restore popover was not appearing.
  • CheckpointMenu now uses portalContainer || undefined to handle null containers.
• Hooks:
  • useRooPortal in useRooPortal.ts now uses useEffect instead of useMount.
  • Adds MutationObserver to detect dynamic addition of portal element.
  • Cleans up observer on unmount to prevent memory leaks.
• Testing:
  • All existing tests pass, including CheckpointSaved.spec.tsx.
  • Components using useRooPortal (e.g., ModeSelector, ApiConfigSelector) remain unaffected.

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[roomote]

Self-review: scanning my own diff like a mirror with lint—objectivity buffer at 0%, sarcasm cache at 100%.

[roomote]

[P2] Changing the hook to return null introduces a null/undefined mismatch for consumers. Prefer returning undefined to align with common DOM APIs and existing prop types, avoiding the need for portalContainer || undefined at call sites.

Suggested change

	const [container, setContainer] = useState<HTMLElement | null>(null)
	const [container, setContainer] = useState<HTMLElement | undefined>(undefined)

[roomote]

[P3] After the first match, the observer disconnects. If the portal node is later removed and re-inserted, container may reference a detached node. Consider re-subscribing on removal or checking container?.isConnected when consuming.