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/js/features/kc-targets.js

@@ -75,6 +75,7 @@ export function createKCTargetCard(target, sourceMap, patternTemplateMap) {
                 <span class="stream-card-prop" title="${t('kc.source')}">📺 ${escapeHtml(sourceName)}</span>
                 <span class="stream-card-prop" title="${t('kc.pattern_template')}">📄 ${escapeHtml(patternName)}</span>
                 <span class="stream-card-prop">▭ ${rectCount} rect${rectCount !== 1 ? 's' : ''}</span>
+                <span class="stream-card-prop" title="${t('kc.fps')}">⚡ ${kcSettings.fps ?? 10} fps</span>
             </div>
             <div class="brightness-control" data-kc-brightness-wrap="${target.id}">
                 <input type="range" class="brightness-slider" min="0" max="255"