Frame interpolation, FPS hot-update, timing metrics, KC brightness fixes

- CSS: add frame interpolation option — blends between consecutive captured
  frames on idle ticks so LED output runs at full target FPS even when
  capture rate is lower (e.g. capture 30fps, output 60fps)
- WledTargetProcessor: re-read stream.target_fps each loop tick so FPS
  changes to the CSS source take effect without restarting the target
- WledTargetProcessor: restore per-stage timing metrics on target card by
  pulling extract/map/smooth/total from CSS stream get_last_timing()
- TargetProcessingState schema: add missing timing_extract_ms,
  timing_map_leds_ms, timing_smooth_ms, timing_total_ms fields
- KC targets: add extraction FPS badge to target card props row
- KC targets: fix 500 error when changing brightness — update_fields now
  accepts (and ignores) WLED-specific kwargs
- KC targets: fix partial key_colors_settings update wiping pattern_template_id
  — update route merges only explicitly-set fields using model_dump(exclude_unset=True)

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

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

@@ -116,6 +116,10 @@ class TargetProcessingState(BaseModel):
     frames_keepalive: Optional[int] = Field(None, description="Keepalive frames sent during standby")
     fps_current: Optional[int] = Field(None, description="Frames sent in the last second")
     timing_send_ms: Optional[float] = Field(None, description="DDP send time (ms)")
+    timing_extract_ms: Optional[float] = Field(None, description="Border pixel extraction time (ms)")
+    timing_map_leds_ms: Optional[float] = Field(None, description="LED color mapping time (ms)")
+    timing_smooth_ms: Optional[float] = Field(None, description="Temporal smoothing time (ms)")
+    timing_total_ms: Optional[float] = Field(None, description="Total processing time per frame (ms)")
     timing_calc_colors_ms: Optional[float] = Field(None, description="Color calculation time (ms, KC targets)")
     timing_broadcast_ms: Optional[float] = Field(None, description="WebSocket broadcast time (ms, KC targets)")
     display_index: Optional[int] = Field(None, description="Current display index")