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

README.md

@@ -29,6 +29,7 @@ and a configurable display.
 | MODE        | 1 / 2 / 3        | BARS / SPECTRUM / LED                         |
 | LEDS        | 4 – 48           | LEDs per bar in LED mode                      |
 | SMOOTH      | 0 – 1            | Waveform smoothing amount in SPECTRUM mode    |
+| GRID        | 0 – 1            | Grid (frequency/level) visibility / brightness|
 
 ## Building