Design system styleguide scaffold

[ephes]

Description

Implement the design-system styleguide scaffold described in specs/2026-01-17_design_system.md. This adds a theme-aware /styleguide/ view, a simple theme switcher, and demo component rendering using shared templates to avoid drift.

Scope / Goals

• Add a theme-aware styleguide view and route, guarded by CAST_ENABLE_STYLEGUIDE.
• Theme switching via ?theme=<name> (styleguide-only; no session persistence).
• Per-theme templates under cast/<theme>/styleguide/ that reuse production partials.
• Demo data via src/cast/devdata.py.
• Smoke tests + basic behavior tests.

Notes

• Dependency: example project fix (specs/2026-01-17_fix_example_project.md).
• Real-world references: ../homepage and ../python-podcast data for component examples.

Acceptance Criteria

• /styleguide/ renders when enabled and returns 404 when disabled.
• ?theme=<name> switches templates; invalid themes fall back with a warning banner.
• Styleguide loads gallery + Podlove components with demo data.
• Tests cover render, theme switching, and disabled behavior.