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/Knob.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <JuceHeader.h>
+#include <memory>
 #include "../Resources.h"
 
 namespace serum
@@ -9,6 +10,10 @@ namespace serum
 // ===========================================================================
 // Rotary knob — a juce::Slider styled by SerumLookAndFeel. The value readout is
 // formatted by a user-provided function via getTextFromValue().
+//
+// While the knob is being dragged it also shows a small floating label near the
+// mouse cursor with the live (formatted) value, giving immediate feedback
+// without changing the slider's normal behaviour.
 // ===========================================================================
 class Knob : public juce::Slider
 {
@@ -20,8 +25,17 @@ public:
 
     juce::String getTextFromValue (double value) override;
 
+    void mouseDown (const juce::MouseEvent& e) override;
+    void mouseDrag (const juce::MouseEvent& e) override;
+    void mouseUp   (const juce::MouseEvent& e) override;
+    void mouseExit (const juce::MouseEvent& e) override;
+
 private:
+    void showValuePopup (const juce::MouseEvent& e);
+    void hideValuePopup();
+
     std::function<juce::String (float)> formatter;
+    std::unique_ptr<juce::Label> valuePopup;
 };
 
 } // namespace serum