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/Analyser.h

@@ -70,5 +70,5 @@ private:
     double falloffRate = 0.8;  // normalized units per second while falling
     double barReleaseTau = 0.2; // seconds for the live bar to fall (release)
     double minDb = -48.0;      // dBFS-equivalent floor (bands below this -> 0)
-    double maxDb = -6.0;       // dBFS-equivalent ceiling (bands at/above -> 1)
+    double maxDb = 0.0;        // dBFS-equivalent ceiling (bands at/above -> 1)
 };