fix(scene): persist the volume type of the "play notification" action

Switching between the Simple and Computed volume tabs only changed the
local state: an action switched to Computed without editing the formula
kept its fixed volume, and one switched back to Simple without moving the
slider kept evaluating the formula.

The tabs now use the same pattern as the "Set a variable" action: clicking
the active tab is a no-op, switching clears the value of the other type,
and switching to Computed seeds the formula with the volume shown by the
slider. They are also real buttons, so they stay reachable with the
keyboard, as the shared valueTypeLink style already documents.

Autofix-Pass: 1

Author: claude

front/src/routes/scene/edit-scene/actions/PlayNotification.jsx

@@ -29,14 +29,37 @@ class PlayNotification extends Component {
       console.error(e);
     }
   };
-  toggleVolumeType = () => this.setState({ computedVolume: !this.state.computedVolume });
+  // Switching the volume type clears the value of the other type, so the action never keeps both
+  // a "volume" and an "evaluate_volume": the server evaluates the formula as soon as it is there,
+  // and the fixed volume would be silently ignored.
+  selectVolumeType = computedVolume => {
+    if (computedVolume === this.state.computedVolume) {
+      return;
+    }
+    this.setState({ computedVolume });
+    if (computedVolume) {
+      // The formula starts from the volume currently selected, so switching to computed does not
+      // lose the value shown by the slider.
+      const { volume } = this.props.action;
+      const evaluateVolume = volume !== undefined ? `${volume}` : undefined;
+      this.props.updateActionProperty(this.props.path, 'evaluate_volume', evaluateVolume);
+      this.props.updateActionProperty(this.props.path, 'volume', undefined);
+    } else {
+      this.props.updateActionProperty(this.props.path, 'evaluate_volume', undefined);
+    }
+  };
+
+  selectSimpleType = () => this.selectVolumeType(false);
+
+  selectComputedType = () => this.selectVolumeType(true);
+
   updateVolume = e => {
     this.props.updateActionProperty(this.props.path, 'volume', parseInt(e.target.value, 10));
     this.props.updateActionProperty(this.props.path, 'evaluate_volume', undefined);
   };
   updateEvaluateVolume = text => {
     this.props.updateActionProperty(this.props.path, 'volume', undefined);
-    this.props.updateActionProperty(this.props.path, 'evaluate_volume', text);
+    this.props.updateActionProperty(this.props.path, 'evaluate_volume', text.length > 0 ? text : undefined);
   };
   updateText = text => {
     this.props.updateActionProperty(this.props.path, 'text', text);
@@ -83,11 +106,7 @@ class PlayNotification extends Component {
           </div>
           <div class="input-group">
             <TextWithVariablesInjected
-              text={
-                this.props.action.volume !== undefined
-                  ? Number(this.props.action.volume).toString()
-                  : this.props.action.evaluate_volume
-              }
+              text={this.props.action.evaluate_volume || ''}
               path={this.props.path}
               triggersVariables={this.props.triggersVariables}
               actionsGroupsBefore={this.props.actionsGroupsBefore}
@@ -155,18 +174,20 @@ class PlayNotification extends Component {
             </span>
           </label>
           <div className={cx('nav-tabs', style.valueTypeTab)}>
-            <span
+            <button
+              type="button"
               class={cx('nav-link', style.valueTypeLink, { active: !this.state.computedVolume })}
-              onClick={this.toggleVolumeType}
+              onClick={this.selectSimpleType}
             >
               <Text id="editScene.actionsCard.playNotification.valueTypeSimple" />
-            </span>
-            <span
+            </button>
+            <button
+              type="button"
               class={cx('nav-link', style.valueTypeLink, { active: this.state.computedVolume })}
-              onClick={this.toggleVolumeType}
+              onClick={this.selectComputedType}
             >
               <Text id="editScene.actionsCard.playNotification.valueTypeComputed" />
-            </span>
+            </button>
           </div>
           {this.getVolumeInput()}
         </div>