feat: Modernisation UI/UX et configuration Flutter multi-plateforme

Phase 1 - Corrections Critiques:
- Fixed memory leaks dans music_provider.dart (stream subscriptions)
- Fixed race conditions dans search_provider.dart (stale results)
- Fixed token refresh errors dans api_service.dart
- Improved error handling avec messages utilisateur
- Changed API URL to HTTPS by default

Phase 2 - Améliorations UX Desktop:
- Ajouté cursor pointers sur tous les éléments cliquables
- Implémenté hover states avec effets néon glow (200ms transitions)
- Créé skeleton loading states avec shimmer animation
- Ajouté widgets: ClickableWrapper, ErrorDisplay, SkeletonLoading
- Enhanced visual feedback pour desktop users

Phase 3 - Configuration Flutter:
- Configuré Android (Gradle 8.1.0, Kotlin 1.9.0, minSdk 21, targetSdk 34)
- Créé launcher icons cyberpunk néon (5 densités)
- Configuré Windows desktop (structure complète)
- Activé Linux desktop support
- Ajouté package équatable pour entités de domaine
- Corrigé imports (colors.dart, auth_provider.dart)
- Fixed Dio API compatibility (RequestOptions)

Documentation:
- STYLE_GUIDE.md: Guide complet (100+ pages)
- DESIGN_IMPLEMENTATION_GUIDE.md: Implémentation Flutter
- BUILD_STATUS.md: Status builds + troubleshooting
- QUICKSTART_BUILDS.md: Guide rapide
- BUILD_INDEX.md: Index documentation
- PHASE_1_CORRECTIONS.md: Corrections Phase 1
- PHASE_2_UX_IMPROVEMENTS.md: Améliorations Phase 2
- PR_REVIEW_SUMMARY.md: Revue code complète
- CODE_ANALYSIS_AND_PRIORITIES.md: Analyse code

Scripts & Builds:
- BUILD_ALL.sh: Script automatisé builds multi-plateforme
- builds/: Structure avec README par plateforme
- design-system/: Système de design complet

Backend:
- Ajouté streaming HTTP Range pour audio progressif
- Enhanced YouTube service avec métadonnées complètes
- Improved error handling et validation

Generated with [Claude Code](https://claude.com/claude-code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

backend/app/main.py

@@ -1,15 +1,21 @@
 """Main FastAPI application entry point."""
 from contextlib import asynccontextmanager
+from pathlib import Path
 from typing import AsyncGenerator
 
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import JSONResponse
+from fastapi.responses import JSONResponse, HTMLResponse
+from fastapi.staticfiles import StaticFiles
 
 from app.core.config import settings
 from app.core.database import close_db, init_db
 
 
+# Get the base directory
+BASE_DIR = Path(__file__).resolve().parent.parent
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     """
@@ -58,15 +64,12 @@ app.add_middleware(
 )
 
 
-@app.get("/")
-async def root() -> dict[str, str]:
-    """Root endpoint with API information."""
-    return {
-        "name": settings.APP_NAME,
-        "version": settings.APP_VERSION,
-        "status": "running",
-        "docs": "/api/docs",
-    }
+@app.get("/", response_class=HTMLResponse)
+async def root() -> str:
+    """Serve the web application."""
+    template_path = BASE_DIR / "app" / "templates" / "index.html"
+    with open(template_path, 'r', encoding='utf-8') as f:
+        return f.read()
 
 
 @app.get("/health")
@@ -112,6 +115,11 @@ app.include_router(auth.router, prefix=settings.API_V1_PREFIX, tags=["authentica
 app.include_router(music.router, prefix=settings.API_V1_PREFIX, tags=["music"])
 app.include_router(playlists.router, prefix=settings.API_V1_PREFIX, tags=["playlists"])
 
+# Mount static files
+static_dir = BASE_DIR / "app" / "static"
+static_dir.mkdir(exist_ok=True)
+app.mount("/static", StaticFiles(directory=str(static_dir)), name="static")
+
 
 if __name__ == "__main__":
     import uvicorn