CSS: add GradientColorStripSource with visual editor

- Backend: GradientColorStripSource storage model, GradientColorStripStream
  with numpy interpolation (bidirectional stops, auto-size from device LED count),
  ColorStop Pydantic schema, API create/update/guard routes
- Frontend: gradient editor modal (canvas preview, draggable markers, stop rows),
  CSS hard-edge card swatch, locale keys (en + ru)
- Fixes: stop row mousedown no longer rebuilds DOM (buttons now clickable),
  position input max-width, bidir/remove button static width

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

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

@@ -8,11 +8,22 @@ from pydantic import BaseModel, Field
 from wled_controller.api.schemas.devices import Calibration
 
 
+class ColorStop(BaseModel):
+    """A single color stop in a gradient."""
+
+    position: float = Field(description="Relative position along the strip (0.0–1.0)", ge=0.0, le=1.0)
+    color: List[int] = Field(description="Primary RGB color [R, G, B] (0–255 each)")
+    color_right: Optional[List[int]] = Field(
+        None,
+        description="Optional right-side RGB color for a hard edge (bidirectional stop)",
+    )
+
+
 class ColorStripSourceCreate(BaseModel):
     """Request to create a color strip source."""
 
     name: str = Field(description="Source name", min_length=1, max_length=100)
-    source_type: Literal["picture", "static"] = Field(default="picture", description="Source type")
+    source_type: Literal["picture", "static", "gradient"] = Field(default="picture", description="Source type")
     # picture-type fields
     picture_source_id: str = Field(default="", description="Picture source ID (for picture type)")
     fps: int = Field(default=30, description="Target frames per second", ge=10, le=90)
@@ -24,6 +35,8 @@ class ColorStripSourceCreate(BaseModel):
     calibration: Optional[Calibration] = Field(None, description="LED calibration (position and count per edge)")
     # static-type fields
     color: Optional[List[int]] = Field(None, description="Static RGB color [R, G, B] (0-255 each, for static type)")
+    # gradient-type fields
+    stops: Optional[List[ColorStop]] = Field(None, description="Color stops for gradient type")
     # shared
     led_count: int = Field(default=0, description="Total LED count (0 = auto from calibration / device)", ge=0)
     description: Optional[str] = Field(None, description="Optional description", max_length=500)
@@ -44,6 +57,8 @@ class ColorStripSourceUpdate(BaseModel):
     calibration: Optional[Calibration] = Field(None, description="LED calibration")
     # static-type fields
     color: Optional[List[int]] = Field(None, description="Static RGB color [R, G, B] (0-255 each, for static type)")
+    # gradient-type fields
+    stops: Optional[List[ColorStop]] = Field(None, description="Color stops for gradient type")
     # shared
     led_count: Optional[int] = Field(None, description="Total LED count (0 = auto from calibration / device)", ge=0)
     description: Optional[str] = Field(None, description="Optional description", max_length=500)
@@ -66,6 +81,8 @@ class ColorStripSourceResponse(BaseModel):
     calibration: Optional[Calibration] = Field(None, description="LED calibration")
     # static-type fields
     color: Optional[List[int]] = Field(None, description="Static RGB color [R, G, B]")
+    # gradient-type fields
+    stops: Optional[List[ColorStop]] = Field(None, description="Color stops for gradient type")
     # shared
     led_count: int = Field(0, description="Total LED count (0 = auto from calibration / device)")
     description: Optional[str] = Field(None, description="Description")