feat(gui): add knob value popup, tooltips, tab cycling, and layout system

Add a floating value label near the cursor while dragging a knob, giving
immediate feedback without altering slider behaviour. Inherit
SettableTooltipClient on ToggleButton so setTooltip works consistently
with knobs and combos.

Introduce a shared layout constants namespace in PluginEditor so every
panel, row and control position derives from one spacing system. Add
tooltips to all knobs, combos, toggles, and buttons. Add Tab/Shift+Tab
tab cycling via keyPressed, with a TooltipWindow for hover hints.

Source/GUI/ToggleButton.h

@@ -9,8 +9,13 @@ namespace serum
 // ===========================================================================
 // LED-style toggle (power) button that drives a float parameter (0/1) or an
 // optional callback (used by RAVE).
+//
+// Inherits SettableTooltipClient so setTooltip() works exactly like it does on
+// juce::Button / juce::Slider / juce::ComboBox (juce::Component itself has no
+// tooltip support).
 // ===========================================================================
-class ToggleButton : public juce::Component
+class ToggleButton : public juce::Component,
+                     public juce::SettableTooltipClient
 {
 public:
     explicit ToggleButton (const juce::String& label = {});