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/static/locales/en.json

@@ -552,6 +552,8 @@
   "color_strip.interpolation.dominant": "Dominant",
   "color_strip.smoothing": "Smoothing:",
   "color_strip.smoothing.hint": "Temporal blending between frames (0=none, 1=full). Reduces flicker.",
+  "color_strip.frame_interpolation": "Frame Interpolation:",
+  "color_strip.frame_interpolation.hint": "Blends between consecutive captured frames to produce output at the full target FPS even when capture rate is lower. Reduces visible stepping on slow ambient transitions.",
   "color_strip.color_corrections": "Color Corrections",
   "color_strip.brightness": "Brightness:",
   "color_strip.brightness.hint": "Output brightness multiplier (0=off, 1=unchanged, 2=double). Applied after color extraction.",