feat(branding): white-label — public /branding, admin write, shell theming (#41)

app_settings branding.* keys (product name, logo/favicon URLs, accent
color, docs/support links). Public GET /branding so login and shell
theme pre-auth; admin writes via PUT /settings.

useBranding() applies document.title, accent CSS var and favicon;
BrandWordmark replaces hardcoded wordmarks; admin Platform settings
gain a Branding form. MCP containr_branding_get + settings_update.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/src/lib/use-branding.ts

@@ -0,0 +1,54 @@
+import { useEffect } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { getBranding, type Branding } from '@/lib/api-client';
+import { useDemoMode } from '@/lib/demo-mode';
+
+const DEFAULT_TITLE = 'containr';
+
+/**
+ * Loads the instance's white-label branding (public endpoint) and applies
+ * it: document title, favicon, and the accent CSS variable. Defaults keep
+ * the app Containr-branded.
+ */
+export function useBranding(): Branding {
+  const isDemoMode = useDemoMode();
+  const brandingQuery = useQuery({
+    queryKey: ['branding'],
+    queryFn: getBranding,
+    staleTime: 5 * 60_000,
+    enabled: !isDemoMode,
+  });
+
+  const branding = brandingQuery.data ?? {
+    productName: 'Containr',
+    logoUrl: '',
+    faviconUrl: '',
+    accentColor: '',
+    docsUrl: '',
+    supportUrl: '',
+  };
+
+  useEffect(() => {
+    const name = branding.productName;
+    document.title = name && name !== 'Containr' ? name : DEFAULT_TITLE;
+
+    const root = document.documentElement;
+    if (branding.accentColor && /^#[0-9a-fA-F]{3,8}$/.test(branding.accentColor)) {
+      root.style.setProperty('--accent-primary', branding.accentColor);
+    } else {
+      root.style.removeProperty('--accent-primary');
+    }
+
+    if (branding.faviconUrl) {
+      let link = document.querySelector<HTMLLinkElement>('link[rel="icon"]');
+      if (!link) {
+        link = document.createElement('link');
+        link.rel = 'icon';
+        document.head.appendChild(link);
+      }
+      link.href = branding.faviconUrl;
+    }
+  }, [branding.productName, branding.accentColor, branding.faviconUrl]);
+
+  return branding;
+}