feat: app shell with sidebar + bug fixes

UI:
- Add persistent sidebar (210px) with band switcher dropdown, Library/Player/Settings nav, user avatar row, and sign-out button
- Align design system CSS vars to CLAUDE.md spec (#0f0f12 bg, #e8a22a amber accent, rgba borders/text)
- Remove light mode toggle (no light mode in v1)
- Homepage auto-redirects to first band; shows create-band form only when no bands exist
- Strip full-page wrappers from all pages (shell owns layout)
- Remove debug console.log statements from SongPage

Bug fixes:
- nginx: trailing slash on `location ^~ /api/v1/bands/` caused 301 redirect on POST, dropping the request body — removed trailing slash
- API: _member_from_request (used by nc-scan stream) only accepted Bearer token, not httpOnly cookie — add rh_token cookie fallback
- API: internal_secret config field now has a dev default so the service starts without INTERNAL_SECRET env var set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/src/rehearsalhub/config.py

@@ -7,7 +7,7 @@ class Settings(BaseSettings):
 
     # Security
     secret_key: str
-    internal_secret: str  # Shared secret for internal service-to-service calls
+    internal_secret: str = "dev-change-me-in-production"  # Shared secret for internal service-to-service calls
     jwt_algorithm: str = "HS256"
     access_token_expire_minutes: int = 60  # 1 hour