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/PluginEditor.h

@@ -30,6 +30,7 @@ public:
     void paint (juce::Graphics&) override;
     void resized() override;
     void timerCallback() override;
+    bool keyPressed (const juce::KeyPress& key) override;
 
     void setTab (int index);
 
@@ -42,6 +43,9 @@ private:
 
     juce::Component root;
 
+    // --- tooltips ---
+    juce::TooltipWindow tooltipWindow;
+
     // --- top bar ---
     std::unique_ptr<juce::Drawable> logo;
     juce::ComboBox presetCombo;
@@ -103,9 +107,12 @@ private:
 
     // --- helpers ---
     Knob* makeKnob (juce::Component* parent, const juce::String& name, const juce::String& paramId,
-                    std::function<juce::String (float)> fmt = {});
-    juce::ComboBox* makeCombo (juce::Component* parent, const juce::String& paramId, const juce::StringArray& items);
-    ToggleButton* makeToggle (juce::Component* parent, const juce::String& label, const juce::String& paramId);
+                    std::function<juce::String (float)> fmt = {}, const juce::String& tooltip = {});
+    juce::ComboBox* makeCombo (juce::Component* parent, const juce::String& paramId, const juce::StringArray& items,
+                               const juce::String& tooltip = {});
+    ToggleButton* makeToggle (juce::Component* parent, const juce::String& label, const juce::String& paramId,
+                              const juce::String& tooltip = {});
+    void cycleTab (int delta);
 
     void buildTopBar();
     void buildOscTab();