Add CLAUDE.md: validate frontend changes across all…

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Record the project rule that any front-end change must be implemented and
validated across the full matrix — weather (on/off and every season),
theme (dark/light), and mode (mobile/desktop) — plus the no-backdrop-filter
fallback. Documents how to set each axis when testing, how validation is
done against the stubbed local build (the sandbox can't reach the live site
or Supabase), and the branch/PR/merge workflow.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pn8V4h9YWbjYzdSrDiLiVL

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