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/SettingsPage.tsx

@@ -363,20 +363,14 @@ export function SettingsPage() {
   const { data: me, isLoading } = useQuery({ queryKey: ["me"], queryFn: getMe });
 
   return (
-    <div style={{ background: "var(--bg)", minHeight: "100vh", color: "var(--text)", padding: 24 }}>
+    <div style={{ padding: 24 }}>
       <div style={{ maxWidth: 540, margin: "0 auto" }}>
-        <div style={{ display: "flex", alignItems: "center", gap: 16, marginBottom: 32 }}>
-          <button
-            onClick={() => navigate("/")}
-            style={{ background: "none", border: "none", color: "var(--text-muted)", cursor: "pointer", fontSize: 13, padding: 0 }}
-          >
-            ← All Bands
-          </button>
-          <h1 style={{ color: "var(--accent)", fontFamily: "monospace", margin: 0, fontSize: 20 }}>Settings</h1>
-        </div>
+        <h1 style={{ color: "var(--text)", margin: "0 0 24px", fontSize: 17, fontWeight: 500 }}>
+          Settings
+        </h1>
 
         {isLoading && <p style={{ color: "var(--text-muted)" }}>Loading...</p>}
-        {me && <SettingsForm me={me} onBack={() => navigate("/")} />}
+        {me && <SettingsForm me={me} onBack={() => navigate(-1)} />}
       </div>
     </div>
   );