Add KC target test button, API docs header link, and UI polish

- Add POST /api/v1/picture-targets/{target_id}/test endpoint for single-frame
  color extraction preview on Key Colors targets
- Add test button on KC target cards that opens lightbox with spinner,
  displays captured frame with rectangle overlays and color swatches
- Add API docs link in WebUI header
- Swap confirm dialog button colors (No=red, Yes=neutral)
- Remove type badges from WLED and KC target cards

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

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

@@ -151,3 +151,23 @@ class TargetMetricsResponse(BaseModel):
     errors_count: int = Field(description="Total error count")
     last_error: Optional[str] = Field(None, description="Last error message")
     last_update: Optional[datetime] = Field(None, description="Last update timestamp")
+
+
+class KCTestRectangleResponse(BaseModel):
+    """A rectangle with its extracted color from a KC test."""
+
+    name: str = Field(description="Rectangle name")
+    x: float = Field(description="Left edge (0.0-1.0)")
+    y: float = Field(description="Top edge (0.0-1.0)")
+    width: float = Field(description="Width (0.0-1.0)")
+    height: float = Field(description="Height (0.0-1.0)")
+    color: ExtractedColorResponse = Field(description="Extracted color for this rectangle")
+
+
+class KCTestResponse(BaseModel):
+    """Response from testing a KC target."""
+
+    image: str = Field(description="Base64 data URI of the captured frame")
+    rectangles: List[KCTestRectangleResponse] = Field(description="Rectangles with extracted colors")
+    interpolation_mode: str = Field(description="Color extraction mode used")
+    pattern_template_name: str = Field(description="Pattern template name")