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.cpp

@@ -21,4 +21,66 @@ juce::String Knob::getTextFromValue (double value)
     return formatPercent ((float) value);
 }
 
+void Knob::mouseDown (const juce::MouseEvent& e)
+{
+    juce::Slider::mouseDown (e);
+    showValuePopup (e);
+}
+
+void Knob::mouseDrag (const juce::MouseEvent& e)
+{
+    juce::Slider::mouseDrag (e);
+    showValuePopup (e);
+}
+
+void Knob::mouseUp (const juce::MouseEvent& e)
+{
+    juce::Slider::mouseUp (e);
+    hideValuePopup();
+}
+
+void Knob::mouseExit (const juce::MouseEvent& e)
+{
+    juce::Slider::mouseExit (e);
+    hideValuePopup();
+}
+
+// ---------------------------------------------------------------------------
+// The floating readout lives as a child of the top-level component so it can
+// follow the cursor outside this knob's own bounds without being clipped, and
+// it never intercepts mouse clicks so it can't block interaction.
+// ---------------------------------------------------------------------------
+void Knob::showValuePopup (const juce::MouseEvent& e)
+{
+    auto* topLevel = getTopLevelComponent();
+    if (topLevel == nullptr)
+        return;
+
+    if (valuePopup == nullptr)
+    {
+        valuePopup = std::make_unique<juce::Label>();
+        valuePopup->setAlwaysOnTop (true);
+        valuePopup->setInterceptsMouseClicks (false, false);
+        valuePopup->setColour (juce::Label::backgroundColourId, juce::Colours::transparentBlack);
+        valuePopup->setColour (juce::Label::textColourId, theme::text);
+        valuePopup->setFont (juce::Font (14.0f, juce::Font::bold));
+        valuePopup->setJustificationType (juce::Justification::centred);
+        topLevel->addAndMakeVisible (valuePopup.get());
+    }
+
+    valuePopup->setText (getTextFromValue (getValue()), juce::dontSendNotification);
+
+    constexpr int w = 84;
+    constexpr int h = 24;
+    constexpr int gap = 10;
+
+    const auto cursor = topLevel->getLocalPoint (nullptr, e.getScreenPosition());
+    valuePopup->setBounds (cursor.getX() - w / 2, cursor.getY() - h - gap, w, h);
+}
+
+void Knob::hideValuePopup()
+{
+    valuePopup.reset();
+}
+
 } // namespace serum

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

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 = {});