fix(banner): position restore-draft banner below app header

[MrCoder]

Summary

• The draft-restoration banner was rendered with position:fixed; top:0, placing it over the app header which contains the diagram title input and Publish button
• Fixed by querying the app header element (header.header) for its actual bottom position via getBoundingClientRect(), falling back to 56px (the h-14 header height) when the element isn't found yet
• Affects all editor variants (sequence, mermaid, openapi, graph, embed) since the banner is shared via restoreDraftBanner.ts

Test plan

• All 665 unit tests pass
• Spot check on staging: open editor, make unsaved change, close, reopen — banner should appear below the header, not over the title/Publish button