fix(wallet): MetaMask Open Snap allowlist messaging + resolveExplorerApiBase

- Clarify stable MetaMask install allowlist vs open Snap permissions on /wallet
- Surface Flask / allowlist-application hint when install errors mention allowlist
- Add shared resolveExplorerApiBase helper for catalog URLs

Made-with: Cursor

frontend/libs/frontend-api-client/api-base.ts

@@ -0,0 +1,25 @@
+const LOCAL_EXPLORER_API_FALLBACK = 'http://localhost:8080'
+
+function normalizeApiBase(value: string | null | undefined): string {
+  return (value || '').trim().replace(/\/$/, '')
+}
+
+export function resolveExplorerApiBase(options: {
+  envValue?: string | null
+  browserOrigin?: string | null
+  serverFallback?: string
+} = {}): string {
+  const explicitBase = normalizeApiBase(options.envValue ?? process.env.NEXT_PUBLIC_API_URL ?? '')
+  if (explicitBase) {
+    return explicitBase
+  }
+
+  const browserOrigin = normalizeApiBase(
+    options.browserOrigin ?? (typeof window !== 'undefined' ? window.location.origin : '')
+  )
+  if (browserOrigin) {
+    return browserOrigin
+  }
+
+  return normalizeApiBase(options.serverFallback ?? LOCAL_EXPLORER_API_FALLBACK)
+}