fix(mobile): enable Metro package-exports resolution for @chat-app/shared

apps/mobile/lib/supabase.ts imports '@chat-app/shared/supabase'. The
shared package declares this subpath in its package.json "exports"
map. The dev Metro resolver respected it; the eager exporter used by
preview/production builds defaulted to legacy resolution and 404'd on
the subpath.

unstable_enablePackageExports=true makes Metro use Node's modern
exports-aware resolver in both modes. Despite the 'unstable_' prefix,
it's the recommended setting in Expo SDK 52 monorepos.

apps/mobile/metro.config.js

@@ -18,5 +18,10 @@ config.resolver.nodeModulesPaths = [
   path.resolve(workspaceRoot, 'node_modules'),
 ];
 config.resolver.disableHierarchicalLookup = true;
+// Make Metro honor the "exports" field in package.json. @chat-app/shared
+// maps subpaths like "./supabase" -> "./src/supabase/index.ts" via exports;
+// without this flag the eager bundler (preview/production) ignores them
+// and falls back to legacy main-only resolution, which 404s on subpaths.
+config.resolver.unstable_enablePackageExports = true;
 
 module.exports = config;