Improve CSS test preview: HD resolution, screen-only border, and refactor frontend docs

- Bump capture preview resolution from 480×360 to 960×540 (HD)
- Increase preview FPS from 2 to ~12 FPS (AUX_INTERVAL 0.5→0.08)
- Add accent-color border on screen rect only (not LED edges) via ::after
- Use dynamic aspect-ratio from decoded JPEG frames instead of fixed height
- Widen modal to 900px for picture sources
- Move frontend conventions from CLAUDE.md to contexts/frontend.md

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

server/src/wled_controller/api/routes/color_strip_sources.py

@@ -775,7 +775,7 @@ async def test_color_strip_ws(
         if is_picture and hasattr(stream, '_live_stream'):
             _frame_live = stream._live_stream
         _last_aux_time = 0.0
-        _AUX_INTERVAL = 0.5  # send JPEG preview / brightness updates every 0.5s
+        _AUX_INTERVAL = 0.08  # send JPEG preview / brightness updates ~12 FPS
 
         # Stream binary RGB frames at ~20 Hz
         while True:
@@ -832,7 +832,7 @@ async def test_color_strip_ws(
                                 img = img[:, :, :3]
                             # Downscale for bandwidth
                             h, w = img.shape[:2]
-                            scale = min(480 / w, 360 / h, 1.0)
+                            scale = min(960 / w, 540 / h, 1.0)
                             if scale < 1.0:
                                 new_w = max(1, int(w * scale))
                                 new_h = max(1, int(h * scale))