Node creation improvements

[kazcw]

Pull Request Description

• The add-component menu button now uses the same placement logic as the enter key (fixes Standardise the logic for positioning a newly added connected component #9703).
• After creating nodes, they are selected and the viewport is adjusted to show them.
• When creating multiple nodes using a fixed-position strategy (such as at the mouse cursor), placements are computed so that newly-created nodes don't occupy the same positions.
• Nodes positioned relative to the mouse are now offset consistently (the new node's icon is centered under the cursor).

Important Notes

• Introduced nodeCreation composable/context-store, which is now the preferred API for creating nodes in contexts
  where the higher-level event-based API is unavailable (which provides a simplified interface by deriving some options
  for the composable from a source-node specification). graphStore's createNode should not generally be used
  directly, as it lacks logic for placement, and doesn't update the focus, selection, and viewport.
• Removed panning computation from usePlacement; we weren't using it. Implemented panning to newly-created nodes using zoomToSelection functionality.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.