Add oscilloscope charge plotting for capacitors

[esaruoho]

Summary

• Adds charge (Q = C*V) as a plottable scope value for capacitors (Feature request: Support viewing the stored charges within the capacitor #202)
• New VAL_CHARGE and UNITS_C constants in Scope.java
• CapacitorElm overrides getScopeValue()/getScopeUnits() for charge display
• ScopePropertiesDialog shows "Show Charge" checkbox when a capacitor is selected
• Also adds charge readout to the capacitor info panel

Test plan

• Place a capacitor in a circuit with an oscilloscope
• Right-click scope → verify "Show Charge" option appears
• Enable charge display → verify Q tracks C*V correctly
• Click on capacitor → verify info panel shows charge value
• Verify no charge option appears for non-capacitor elements

Fixes #202

🤖 Generated with Claude Code

[pfalstad]

When you check "Show Charge" it doesn't show the voltage/current anymore but they are still checked.

Then if you uncheck Voltage and check it again, it shows the voltage and not the charge, even though charge is still checked.

[esaruoho]

Fixed charge plotting toggle — Show Charge now works independently alongside Show Voltage and Show Current. All three can be displayed simultaneously without interfering with each other.

[esaruoho]

Fixed charge plotting toggle:

The root cause was that handleMenu(showcharge) called setValue(VAL_CHARGE), which replaced the entire plots vector -- destroying voltage and current plots.

Now uses a new showCharge(boolean) method that additively adds/removes the charge plot without affecting other plots. Also updated showingVoltageAndMaybeCurrent() to allow charge alongside voltage/current, preventing the destroy-on-recheck behavior.

All three (voltage, current, charge) can now be displayed simultaneously and toggled independently.