feat(frontend): scaffold Flutter Web app with ENSUP home screen

eme-frontend/lib/theme/ensup_theme.dart

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+import "package:flutter/material.dart";
+import "package:google_fonts/google_fonts.dart";
+import "ensup_colors.dart";
+
+/// Thème global : couleurs ENSUP et police de corps Titillium Web.
+/// Les titres utilisent Darker Grotesque, appliqué directement dans les widgets.
+ThemeData buildEnsupTheme() {
+  final base = ThemeData(
+    useMaterial3: true,
+    colorScheme: ColorScheme.fromSeed(
+      seedColor: EnsupColors.cyan,
+      primary: EnsupColors.cyan,
+    ),
+    scaffoldBackgroundColor: EnsupColors.soft,
+  );
+
+  return base.copyWith(
+    textTheme: GoogleFonts.titilliumWebTextTheme(base.textTheme).apply(
+      bodyColor: EnsupColors.text,
+      displayColor: EnsupColors.text,
+    ),
+  );
+}