Add GRID visibility knob and fix spectrum/grid rendering

- Add GRID rotary knob (0-1) controlling grid + axis-label brightness/visibility
- Raise analyser ceiling to 0 dBFS; remove overlapping 0/-6 dB lines
- Set plugin minimum width to 800px (and clamp restored width)
- Extend spectrum signal/peak waves to full plot width
- Render signal wave as quadratic spline to match peak smoothness
- Restore 1px gaps in LED mode

Source/PluginEditor.h

@@ -30,6 +30,7 @@ private:
     juce::Slider modeSlider;
     juce::Slider ledsSlider;
     juce::Slider smoothSlider;
+    juce::Slider gridSlider;
 
     juce::Label graceLabel;
     juce::Label falloffLabel;
@@ -39,6 +40,7 @@ private:
     juce::Label modeLabel;
     juce::Label ledsLabel;
     juce::Label smoothLabel;
+    juce::Label gridLabel;
     juce::Label titleLabel;
     juce::Label statusLabel;
 
@@ -52,6 +54,7 @@ private:
     std::unique_ptr<juce::AudioProcessorValueTreeState::SliderAttachment> modeAttachment;
     std::unique_ptr<juce::AudioProcessorValueTreeState::SliderAttachment> ledsAttachment;
     std::unique_ptr<juce::AudioProcessorValueTreeState::SliderAttachment> smoothAttachment;
+    std::unique_ptr<juce::AudioProcessorValueTreeState::SliderAttachment> gridAttachment;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (BeamgridAudioProcessorEditor)
 };