fix: prevent path editor reload and layout shift in summary popover

[Guilhem-lm]

Summary

Fix two issues in the SummaryPathDisplay popover related to the Path component.

Changes

• Wrap $effect body in untrack() so only popoverOpen is tracked — prevents the Path component from reinitializing on every keystroke in the summary input
• Add a fixed-height placeholder with inline loader in Path.svelte while meta is loading, preventing layout shift when the path editor appears

Test plan

• Open a flow/script, click the summary/path area to open the popover
• Type in the summary input — the path editor should NOT reload
• Verify the popover does not shift when the path editor loads

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