Add configurable FPS to test preview and fix composite stream release race

- Add FPS control (1-60, default 20) to test preview modal next to LED count
- Server accepts fps query param, controls frame send interval
- Single Apply icon button (✓) applies both LED count and FPS
- FPS control stays visible for picture sources (LED count hidden)
- Fix composite sub-stream consumer ID collision: use unique instance ID
  to prevent old WebSocket release from killing new connection's streams

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

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

@@ -42,11 +42,16 @@
                 <div id="css-test-layers" class="css-test-layers"></div>
             </div>
 
-            <!-- LED count control -->
-            <div class="css-test-led-control" id="css-test-led-control">
-                <label for="css-test-led-input" data-i18n="color_strip.test.led_count">LEDs:</label>
-                <input type="number" id="css-test-led-input" min="1" max="2000" step="1" value="100" class="css-test-led-input">
-                <button class="btn btn-sm btn-secondary css-test-led-apply" onclick="applyCssTestLedCount()" data-i18n="color_strip.test.apply">Apply</button>
+            <!-- LED count & FPS controls -->
+            <div class="css-test-led-control">
+                <span id="css-test-led-group">
+                    <label for="css-test-led-input" data-i18n="color_strip.test.led_count">LEDs:</label>
+                    <input type="number" id="css-test-led-input" min="1" max="2000" step="1" value="100" class="css-test-led-input">
+                    <span class="css-test-separator"></span>
+                </span>
+                <label for="css-test-fps-input" data-i18n="color_strip.test.fps">FPS:</label>
+                <input type="number" id="css-test-fps-input" min="1" max="60" step="1" value="20" class="css-test-led-input">
+                <button class="btn btn-icon btn-sm btn-secondary css-test-led-apply" onclick="applyCssTestSettings()" title="Apply" data-i18n-title="color_strip.test.apply">&#x2713;</button>
             </div>
 
             <div id="css-test-status" class="css-test-status" data-i18n="color_strip.test.connecting">Connecting...</div>