Fix nonlocal scoping in CSS processing, move brightness source emoji to dropdown items

Add nonlocal declarations for _u16_a, _u16_b, _i32 in nested functions
_blend_u16 and _apply_corrections — Python treats augmented assignments
(*=, +=, >>=) as local variable bindings, causing UnboundLocalError
that prevented any frames from being sent to devices.

Move 🔢 emoji from brightness source label to dropdown option items.

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

server/src/wled_controller/core/processing/color_strip_stream.py

@@ -305,6 +305,7 @@ class PictureColorStripStream(ColorStripStream):
 
             Uses pre-allocated uint16 scratch buffers (_u16_a, _u16_b).
             """
+            nonlocal _u16_a, _u16_b
             np.copyto(_u16_a, a, casting='unsafe')
             np.copyto(_u16_b, b, casting='unsafe')
             _u16_a *= (256 - alpha_b)
@@ -318,6 +319,7 @@ class PictureColorStripStream(ColorStripStream):
 
             Returns the (possibly reassigned) led_colors array.
             """
+            nonlocal _i32
             if self._saturation != 1.0:
                 _apply_saturation(led_colors, self._saturation, _i32, _i32_gray, led_colors)
             if self._gamma != 1.0: