fix: Vue Node <-> Litegraph node height offset normalization (#6966)

## Summary

Changes the layout store to treat node sizes as body-only measurements
while LiteGraph continues to reason about full heights. DOM-driven
updates are tagged with `LayoutSource.DOM`, which lets the store strip
the title height exactly once before persisting. That classification (a
new mutation source - `LayoutSource.DOM`) is accurate because those
mutations are triggered by the browser’s layout engine via
ResizeObserver, rather than by direct calls into the layout APIs (e.g.,
`moveNodeTo`, `useNodeDrag`). So all sources are:

- `LayoutSource.DOM`: browser layout/ResizeObserver measurements that
include the title bar
- `LayoutSource.Vue`: direct Vue-driven mutations routed through the
layout store
- `LayoutSource.Canvas`: legacy LiteGraph/canvas updates that will be
phased out over time
- `LayoutSource.External`: for multiplayer or syncing with a when going
online after making changes offline (in teams/workspace)

When layout state flows back into LiteGraph we add the title height just
in time for `liteNode.setSize`, so LiteGraph’s rendering stays
unchanged. This makes Vue node resizing and workflow persistence
deterministic - multiline widgets hold their dimensions across reloads
because every path that crosses the layout/LiteGraph boundary performs
the same normalization.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6966-normalize-height-at-sites-2b76d73d365081b6bcb4f4ce6a27663a)
by [Unito](https://www.unito.io)

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Co-authored-by: github-actions <[email protected]>

Author: christian-byrne