feat: new value source types (HA entity, gradient map, strip extract) + UI fixes

New value source types:
- ha_entity: reads numeric values from HA entity state/attribute, normalizes
  via min/max range, applies EMA smoothing. EntitySelect for HA connection
  and entity selection with live entity list fetching.
- gradient_map: maps a float value source (0-1) through a gradient entity.
  EntitySelect for both input source and gradient with inline previews.
- css_extract: extracts single color by averaging LED range from a color
  strip source. EntitySelect for source selection.

Value source type picker:
- Filter tabs (All / Numeric / Color) above the icon grid
- showTypePicker extended with filterTabs + onFilterChange support

Palette selectors converted to EntitySelect:
- Effect palette, gradient preset, and audio palette selectors now use
  command-palette style EntitySelect with gradient strip previews

Tab indicator fixes:
- Icon now updates on tab switch (was passing no args to updateTabIndicator)
- Visible with any background effect active, not just Noise Field
- Noise Field is the default background effect for new users

Dashboard section collapse fix:
- Split header into clickable toggle (chevron+label) and non-clickable
  actions area — buttons no longer trigger collapse/expand

Discriminated union fix (422 errors):
- source_type/target_type now always included in update payloads for:
  CSS editor, LED target, HA light target, simple calibration,
  advanced calibration

server/src/wled_controller/templates/modals/ha-light-editor.html

@@ -60,14 +60,14 @@
                     <div id="ha-light-editor-transition-container"></div>
                 </div>
 
-                <!-- Brightness Value Source -->
+                <!-- Brightness -->
                 <div class="form-group">
                     <div class="label-row">
-                        <label for="ha-light-editor-brightness-vs" data-i18n="targets.brightness_vs">Brightness Source:</label>
+                        <label data-i18n="targets.brightness">Brightness:</label>
+                        <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
                     </div>
-                    <select id="ha-light-editor-brightness-vs">
-                        <option value="">None</option>
-                    </select>
+                    <small class="input-hint" style="display:none" data-i18n="targets.brightness.hint">Output brightness multiplier (0–1). Can be bound to a value source for dynamic control.</small>
+                    <div id="ha-light-editor-brightness-container"></div>
                 </div>
 
                 <!-- Color Tolerance -->