feat(design): v2 becomes the default — drop beta conversation

Atmospheric design is now the default for everyone:
- User.getDesignVersion() returns 'v2' when unset (was 'v1')
- All Twig templates default the cookie read to 'v2'
- SettingsView 'Design (beta)' section removed entirely along with the
  selectDesign() handler and currentDesign computed

The /api/user/design endpoint stays in place so the design_version column
can still be set programmatically (or migrated later), but the UI no
longer exposes it as a user-facing choice. Existing users with explicit
'v1' in their DB row continue to see v1 — their preference persists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

public/build/index.html

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