feat(navi): config-driven tile source, defaults, and feature flags

Load deployment config from /api/config on startup:
- src/config.js: loader with 3s timeout + hardcoded fallback
- src/hooks/useConfig.js: useConfig() and useFeature() hooks
- MapView.jsx: tile URL, attribution, center, zoom from config
- main.jsx: loads config before first render

Falls back to home profile defaults if backend unavailable.
No visible behavior change — infrastructure for future features.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useConfig.js

@@ -0,0 +1,29 @@
+import { useState, useEffect } from 'react'
+import { loadConfig, getConfig, hasFeature } from '../config'
+
+/**
+ * Hook that returns the deployment config, loading it if needed.
+ * Components using this will re-render once config is loaded.
+ */
+export function useConfig() {
+  const [config, setConfig] = useState(getConfig)
+
+  useEffect(() => {
+    if (!config) {
+      loadConfig().then(setConfig)
+    }
+  }, [config])
+
+  return config
+}
+
+/**
+ * Hook to check a single feature flag.
+ * @param {string} flag - e.g. 'has_hillshade'
+ * @returns {boolean}
+ */
+export function useFeature(flag) {
+  const config = useConfig()
+  if (!config) return false
+  return Boolean(config.features?.[flag])
+}