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/templates/modals/css-editor.html

@@ -78,6 +78,15 @@
                         <input type="range" id="css-editor-smoothing" min="0.0" max="1.0" step="0.05" value="0.3" oninput="document.getElementById('css-editor-smoothing-value').textContent = parseFloat(this.value).toFixed(2)">
                     </div>
 
+                    <div class="form-group">
+                        <div class="label-row">
+                            <label for="css-editor-frame-interpolation" data-i18n="color_strip.frame_interpolation">Frame Interpolation:</label>
+                            <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
+                        </div>
+                        <small class="input-hint" style="display:none" data-i18n="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.</small>
+                        <input type="checkbox" id="css-editor-frame-interpolation">
+                    </div>
+
                     <details class="form-collapse">
                         <summary data-i18n="color_strip.color_corrections">Color Corrections</summary>
                         <div class="form-collapse-body">