chore: Remove index hack, move canvas layout outside for readability

[istarkov]

Description

iframe now points to /canvas route
$ (splat) route has no UI, and now does only technical part.
_index route will be builder route on builder urls i.e. {ProjectId}.apps.webstudio.is (next PR)

Steps for reproduction

1. click button
2. expect xyz

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 5de6)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file

[kof]

approving, will do some more testing later