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/index.css

@@ -4,7 +4,7 @@
 
 body {
   margin: 0;
-  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
+  font-family: -apple-system, BlinkMacSystemFont, "Helvetica Neue", sans-serif;
   -webkit-font-smoothing: antialiased;
   background: var(--bg);
   color: var(--text);
@@ -14,39 +14,23 @@ input, textarea, button, select {
   font-family: inherit;
 }
 
-/* ── Dark theme (default) — GitHub Primer slate ─────────────────────────── */
-:root,
-[data-theme="dark"] {
-  --bg:          #0d1117;
-  --bg-subtle:   #161b22;
-  --bg-inset:    #21262d;
-  --border:      #30363d;
-  --text:        #e6edf3;
-  --text-muted:  #8b949e;
-  --text-subtle: #484f58;
-  --accent:      #f0a840;
-  --accent-bg:   #2a1e08;
-  --accent-fg:   #080c10;
-  --teal:        #38c9a8;
-  --teal-bg:     #0a2820;
-  --danger:      #f85149;
-  --danger-bg:   #1a0810;
-}
-
-/* ── Light theme ─────────────────────────────────────────────────────────── */
-[data-theme="light"] {
-  --bg:          #ffffff;
-  --bg-subtle:   #f6f8fa;
-  --bg-inset:    #eef1f5;
-  --border:      #d0d7de;
-  --text:        #24292f;
-  --text-muted:  #57606a;
-  --text-subtle: #afb8c1;
-  --accent:      #f0a840;
-  --accent-bg:   #fff8e1;
-  --accent-fg:   #1c1007;
-  --teal:        #0a6b56;
-  --teal-bg:     #d1f7ef;
-  --danger:      #cf222e;
-  --danger-bg:   #ffebe9;
+/* ── Design system (dark only — no light mode in v1) ─────────────────────── */
+:root {
+  --bg:          #0f0f12;
+  --bg-subtle:   rgba(255,255,255,0.025);
+  --bg-inset:    rgba(255,255,255,0.04);
+  --border:      rgba(255,255,255,0.08);
+  --border-subtle: rgba(255,255,255,0.05);
+  --text:        #eeeef2;
+  --text-muted:  rgba(255,255,255,0.35);
+  --text-subtle: rgba(255,255,255,0.22);
+  --accent:      #e8a22a;
+  --accent-hover: #f0b740;
+  --accent-bg:   rgba(232,162,42,0.1);
+  --accent-border: rgba(232,162,42,0.28);
+  --accent-fg:   #0f0f12;
+  --teal:        #4dba85;
+  --teal-bg:     rgba(61,200,120,0.1);
+  --danger:      #e07070;
+  --danger-bg:   rgba(220,80,80,0.1);
 }