Add value source test modal, auto-gain, brightness always-show, shared value streams

- Add real-time value source test: WebSocket endpoint streams get_value() at
  ~20Hz, frontend renders scrolling time-series chart with min/max/current stats
- Add auto-gain for audio value sources: rolling peak normalization with slow
  decay, sensitivity range increased to 0.1-20.0
- Always show brightness overlay on LED preview when brightness source is set
- Refactor ValueStreamManager to shared ref-counted streams (value streams
  produce scalars, not LED-count-dependent, so sharing is correct)
- Simplify acquire/release API: remove consumer_id parameter since streams
  are no longer consumer-dependent

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

server/src/wled_controller/templates/modals/test-value-source.html

@@ -0,0 +1,27 @@
+<!-- Test Value Source Modal -->
+<div id="test-value-source-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="test-value-source-modal-title">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 id="test-value-source-modal-title" data-i18n="value_source.test.title">Test Value Source</h2>
+            <button class="modal-close-btn" onclick="closeTestValueSourceModal()" title="Close" data-i18n-aria-label="aria.close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <canvas id="vs-test-canvas" class="vs-test-canvas"></canvas>
+            <div class="vs-test-stats">
+                <span class="vs-test-stat vs-test-stat-current">
+                    <span class="vs-test-stat-label" data-i18n="value_source.test.current">Current</span>
+                    <span class="vs-test-stat-value vs-test-value-large" id="vs-test-current">---</span>
+                </span>
+                <span class="vs-test-stat">
+                    <span class="vs-test-stat-label" data-i18n="value_source.test.min">Min</span>
+                    <span class="vs-test-stat-value" id="vs-test-min">---</span>
+                </span>
+                <span class="vs-test-stat">
+                    <span class="vs-test-stat-label" data-i18n="value_source.test.max">Max</span>
+                    <span class="vs-test-stat-value" id="vs-test-max">---</span>
+                </span>
+            </div>
+            <div id="vs-test-status" class="vs-test-status" data-i18n="value_source.test.connecting">Connecting...</div>
+        </div>
+    </div>
+</div>