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>

web/src/pages/SessionPage.tsx

@@ -60,13 +60,13 @@ export function SessionPage() {
   if (!session) return <div style={{ color: "var(--danger)", padding: 32 }}>Session not found</div>;
 
   return (
-    <div style={{ background: "var(--bg)", minHeight: "100vh", color: "var(--text)", padding: 32 }}>
+    <div style={{ padding: 32 }}>
       <div style={{ maxWidth: 720, margin: "0 auto" }}>
         <Link
           to={`/bands/${bandId}`}
           style={{ color: "var(--text-muted)", fontSize: 12, textDecoration: "none", display: "inline-block", marginBottom: 20 }}
         >
-          ← Back to Band
+          ← Library
         </Link>
 
         {/* Header */}