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/config.js

@@ -0,0 +1,84 @@
+/**
+ * Deployment config loader.
+ *
+ * Fetches /api/config on startup and caches the result.
+ * Falls back to hardcoded defaults matching the home profile if the
+ * API is unavailable (backend restart, network issue).
+ */
+
+const FALLBACK_CONFIG = {
+  profile: 'home',
+  region_name: 'North America',
+  tileset: {
+    url: '/tiles/na.pmtiles',
+    bounds: [-168, 14, -52, 72],
+    max_zoom: 15,
+    attribution: 'Protomaps © OSM',
+  },
+  services: {
+    geocode: '/api/geocode',
+    reverse: '/api/reverse',
+    address_book: '/api/address_book',
+    valhalla: '/valhalla',
+  },
+  features: {
+    has_nominatim_details: false,
+    has_kiwix_wiki: false,
+    has_hillshade: false,
+    has_3d_terrain: false,
+    has_traffic_overlay: false,
+    has_landclass: false,
+    has_address_book_write: false,
+  },
+  defaults: {
+    center: [42.5736, -114.6066],
+    zoom: 10,
+  },
+}
+
+let _config = null
+let _configPromise = null
+
+/**
+ * Fetch config from backend. Returns cached config on subsequent calls.
+ * Falls back to FALLBACK_CONFIG if API fails.
+ */
+export function loadConfig() {
+  if (_configPromise) return _configPromise
+
+  _configPromise = fetch('/api/config', { signal: AbortSignal.timeout(3000) })
+    .then((resp) => {
+      if (!resp.ok) throw new Error(`Config API returned ${resp.status}`)
+      return resp.json()
+    })
+    .then((data) => {
+      _config = data
+      console.log('[navi] Config loaded:', data.profile, `(${data.region_name})`)
+      console.log('[navi] Feature flags:', data.features)
+      return data
+    })
+    .catch((err) => {
+      console.warn('[navi] Config API unavailable, using fallback:', err.message)
+      _config = FALLBACK_CONFIG
+      return FALLBACK_CONFIG
+    })
+
+  return _configPromise
+}
+
+/**
+ * Get the current config synchronously. Returns null if not yet loaded.
+ */
+export function getConfig() {
+  return _config
+}
+
+/**
+ * Check a feature flag from the loaded config.
+ * @param {string} flag - Feature flag name (e.g. 'has_hillshade')
+ * @returns {boolean}
+ */
+export function hasFeature(flag) {
+  if (!_config) return false
+  return Boolean(_config.features?.[flag])
+}