feat(gui): add GUI components and displays

Source/GUI/ToggleButton.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <JuceHeader.h>
+#include "../Resources.h"
+
+namespace serum
+{
+
+// ===========================================================================
+// LED-style toggle (power) button that drives a float parameter (0/1) or an
+// optional callback (used by RAVE).
+// ===========================================================================
+class ToggleButton : public juce::Component
+{
+public:
+    explicit ToggleButton (const juce::String& label = {});
+
+    void attach (juce::AudioProcessorValueTreeState& apvts, const juce::String& paramId);
+    void setOnClick (std::function<void (bool)> cb) { onClick = std::move (cb); }
+
+    void setOnColour (juce::Colour c) { onColour = c; repaint(); }
+    void setLabel (const juce::String& lbl) { label = lbl; repaint(); }
+
+    void setToggleState (bool s);
+    bool getToggleState() const noexcept { return state; }
+
+    void paint (juce::Graphics& g) override;
+    void mouseDown (const juce::MouseEvent&) override;
+
+private:
+    bool        state = false;
+    juce::String label;
+    juce::RangedAudioParameter* param = nullptr;
+    std::unique_ptr<juce::ParameterAttachment> attachment;
+    std::function<void (bool)> onClick;
+    juce::Colour onColour = theme::accent;
+};
+
+} // namespace serum