Split monolithic index.html and style.css for maintainability

- Extract 15 modals and 3 partials from index.html into Jinja2 templates
  (templates/modals/*.html, templates/partials/*.html)
- Split style.css (3,712 lines) into 11 feature-scoped CSS files under
  static/css/ (base, layout, components, cards, modal, calibration,
  dashboard, streams, patterns, profiles, tutorials)
- Switch root route from FileResponse to Jinja2Templates
- Add jinja2 dependency
- Consolidate duplicate @keyframes spin definition
- Browser receives identical assembled HTML — zero JS changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/main.py

@@ -6,8 +6,10 @@ from pathlib import Path
 
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import JSONResponse, FileResponse
+from fastapi.responses import JSONResponse
 from fastapi.staticfiles import StaticFiles
+from fastapi.templating import Jinja2Templates
+from starlette.requests import Request
 
 from wled_controller import __version__
 from wled_controller.api import router
@@ -262,6 +264,10 @@ if static_path.exists():
 else:
     logger.warning(f"Static files directory not found: {static_path}")
 
+# Jinja2 templates
+templates_path = Path(__file__).parent / "templates"
+templates = Jinja2Templates(directory=str(templates_path))
+
 
 @app.exception_handler(Exception)
 async def global_exception_handler(request, exc):
@@ -279,20 +285,9 @@ async def global_exception_handler(request, exc):
 
 
 @app.get("/")
-async def root():
+async def root(request: Request):
     """Serve the web UI dashboard."""
-    static_path = Path(__file__).parent / "static" / "index.html"
-    if static_path.exists():
-        return FileResponse(static_path)
-
-    # Fallback to JSON if static files not found
-    return {
-        "name": "LED Grab",
-        "version": __version__,
-        "docs": "/docs",
-        "health": "/health",
-        "api": "/api/v1",
-    }
+    return templates.TemplateResponse(request, "index.html")
 
 
 if __name__ == "__main__":