M6-04: Add browser regression tests

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L814
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:test, area:viewer

• Use Playwright to test:
  • viewer loads
  • dropzone visible
  • demo/sample load path
  • canvas nonblank after model load
  • category toggles work
  • element info panel opens
  • export buttons enable/disable correctly
  • diagnostics show partial-export status

Acceptance criteria:

• Tests run in CI for viewer PRs.
• Screenshots are captured on failure.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.