feat: Phase 1 — FIT/GPX/TCX extraction via Pyodide in hidden WebView

- extraction/PyodideWebView.tsx: hidden WebView (1×1 px, off-screen) that
  bootstraps Pyodide v0.26.4 from jsDelivr CDN on app startup; loads lxml,
  pyyaml, micropip, fitdecode, gpxpy automatically; installs the bincio wheel
  lazily on the first extraction call via a blob URL (avoids startup delay)
- extraction/extractActivity.ts: typed bridge — extractFile(filename, base64,
  wheelUrl, onStatus) injects JS into the WebView, tracks pending promises by
  request ID, resolves with { id, detail, timeseries, geojson, sourceHash }
- app/_layout.tsx: mounts <PyodideWebView> outside SQLiteProvider at root so
  the runtime warms up as soon as the app opens
- app/(tabs)/import.tsx: replaces the placeholder alert with real extraction;
  reads files as base64, calls extractFile with a progress callback, stores
  detail_json + timeseries_json + geojson + real SHA-256 source_hash; resolves
  wheel URL via GET /api/wheel/version with fallback to /api/wheel/download;
  falls back to bincio.org if no instance is configured

mobile/app/_layout.tsx

@@ -2,12 +2,18 @@ import { Stack } from 'expo-router';
 import { SQLiteProvider } from 'expo-sqlite';
 import { StatusBar } from 'expo-status-bar';
 import { migrateDb } from '@/db';
+import { PyodideWebView } from '@/extraction/PyodideWebView';
 
 export default function RootLayout() {
   return (
-    <SQLiteProvider databaseName="bincio.db" onInit={migrateDb}>
-      <StatusBar style="light" />
-      <Stack screenOptions={{ headerShown: false }} />
-    </SQLiteProvider>
+    <>
+      {/* Hidden WebView: starts loading Pyodide immediately so the runtime
+          is warm by the time the user opens the Import tab. */}
+      <PyodideWebView />
+      <SQLiteProvider databaseName="bincio.db" onInit={migrateDb}>
+        <StatusBar style="light" />
+        <Stack screenOptions={{ headerShown: false }} />
+      </SQLiteProvider>
+    </>
   );
 }