feat: Expo mobile scaffold — Phase 2 complete (118-file structure)

Expo SDK 51 TypeScript scaffold with all architecture files.
Verified: tsc 0 errors, jest passes.

mobile/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "extends": "expo/tsconfig.base",
+  "compilerOptions": {
+    "strict": true,
+    "baseUrl": ".",
+    "paths": {
+      "@/*": ["./src/*"]
+    }
+  }
+}