feat(web): init feature flags + kill switch on startup, fix hardcoded colors

- Providers.tsx now calls initFeatureFlags() and checkKillSwitch()
  on mount (both were wired but never initialized).
- globals.css: replace hardcoded #0b1020 with color-mix() from canvas token.
- layout.tsx: make themeColor responsive (dark/light media queries).
- use-theme.ts: prefix localStorage key with PRODUCT_ID for consistency.

web/src/app/globals.css

@@ -14,7 +14,7 @@ html {
 body {
   margin: 0;
   min-height: 100vh;
-  background: linear-gradient(180deg, var(--nl-bg-canvas) 0%, #0b1020 100%);
+  background: linear-gradient(180deg, var(--nl-bg-canvas) 0%, color-mix(in srgb, var(--nl-bg-canvas) 70%, black) 100%);
   color: var(--nl-text-primary);
   font-family: var(--nl-font-body);
   -webkit-font-smoothing: antialiased;

web/src/app/layout.tsx

@@ -24,7 +24,10 @@ export const metadata: Metadata = {
 export const viewport: Viewport = {
   width: "device-width",
   initialScale: 1,
-  themeColor: "#06070A",
+  themeColor: [
+    { media: "(prefers-color-scheme: dark)", color: "#06070A" },
+    { media: "(prefers-color-scheme: light)", color: "#F8FAFC" },
+  ],
 };
 
 export default function RootLayout({

web/src/app/providers.tsx

@@ -6,12 +6,16 @@ import { Toaster } from "sonner";
 import { AuthProvider } from "@/lib/auth";
 import { initDiagnostics } from "@/lib/diagnostics";
 import { initTelemetry } from "@/lib/telemetry";
+import { initFeatureFlags } from "@/lib/feature-flags";
+import { checkKillSwitch } from "@/lib/kill-switch";
 import { flushOfflineQueue } from "@/lib/offline-queue";
 
 export function Providers({ children }: { children: ReactNode }) {
   useEffect(() => {
     initTelemetry();
     initDiagnostics();
+    initFeatureFlags().catch(() => {});
+    checkKillSwitch().catch(() => {});
     flushOfflineQueue().catch(() => {});
   }, []);
 

web/src/lib/use-theme.ts

@@ -1,14 +1,16 @@
 'use client';
 
 import { useState, useEffect, useCallback } from 'react';
+import { PRODUCT_ID } from '@/lib/product-config';
 
 type Theme = 'dark' | 'light';
+const STORAGE_KEY = `${PRODUCT_ID}_theme`;
 
 export function useTheme() {
   const [theme, setThemeState] = useState<Theme>('dark');
 
   useEffect(() => {
-    const stored = localStorage.getItem('theme') as Theme | null;
+    const stored = localStorage.getItem(STORAGE_KEY) as Theme | null;
     const initial = stored ?? 'dark';
     setThemeState(initial);
     document.documentElement.setAttribute('data-theme', initial);
@@ -16,7 +18,7 @@ export function useTheme() {
 
   const setTheme = useCallback((t: Theme) => {
     setThemeState(t);
-    localStorage.setItem('theme', t);
+    localStorage.setItem(STORAGE_KEY, t);
     document.documentElement.setAttribute('data-theme', t);
   }, []);