Website | Gallery: Fix expandable sankey data model

[reb-dev]

#180

Background:
The error was occurring because data.nodes and data.links weren't in sync. Previously, only the node objects were updated when setting a node's expanded property, while links remained unchanged, resulting in links with unrecognized source and target values when Sankey wasn't fully expanded. (I suspect changes from Datamodel | Graph: Fix link data logic (8b4416d) are what ultimately broke it)

Changes :
This PR:

• Updates both nodes and links whenever a node's subtree is toggled
• Extracts expand/collapse logic to data.ts file to maintain clarity in the component code
• Changes the data shape and type defs a bit to be more readable
• Includes minor tweaks to increase consistency across example files

[rokotyan]

@reb-dev Thanks for fixing the problem and explaining the reason, appreciate it.