view: tighten TypeScript types in web/view

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Found in the review of #2057, which adds the experimental view. Paths refer to #2059's head.

Summary

TypeScript type safety in web/view/src/:

Composite string keys; see view: node ids are ::-joined strings that the page splits again #2061.
Unchecked as casts in main.ts.
The as Record cast of CLASSES in graphcalLanguage.ts.
DOM classes used as state.
previousValue gives undefined a meaning of its own.
CSS selectors built from data without CSS.escape.