fix scope charge checkbox toggling

[esaruoho]

What this fixes (beyond #248)

PR #248 added charge plotting and fixed the initial issue you reported (Show Charge hiding voltage/current). However, the showCharge() implementation directly adds/removes plots from the list, unlike showVoltage()/showCurrent() which use boolean flags (showV/showI) filtered by calcVisiblePlots(). This mismatch causes three remaining bugs:

1. setValue() destroys charge plots. Toggling scope values (e.g. clicking "Show Resistance") calls setValue(), which rebuilds the plots vector from scratch. Charge plots are lost because addValue() doesn't know to re-add them. With a showC flag, addValue() can restore the charge plot after a rebuild.

2. Charge state not persisted on save/load. getFlags()/setFlags() don't include charge state, so saving a circuit and reloading it loses the "Show Charge" setting.

3. No visibility toggle — only add/destroy. Unchecking "Show Charge" removes the plot entirely and calls resetGraph(), destroying accumulated data. Voltage and current instead just toggle visibility via showV/showI without removing the underlying plot data.

Fix

Adds a showC boolean flag that mirrors the existing showV/showI pattern:

• showCharge() sets showC and adds the plot if needed, but no longer removes plots or resets the graph
• calcVisiblePlots() filters UNITS_C plots by showC (like UNITS_V/showV and UNITS_A/showI)
• addValue() re-adds the charge plot when showC is true after a setValue() rebuild
• initialize() detects existing charge plots and sets showC
• getFlags()/setFlags() persist showC using bit 1<<23
• ScopePropertiesDialog.updateUi() reads scope.showC instead of scope.hasPlotValue()

Steps to reproduce the bugs (on current v3-dev)

1. Place a capacitor with a scope → enable Show Charge → enable Show Resistance → charge plot disappears (bug 1)
2. Place a capacitor with a scope → enable Show Charge → save circuit → reload → charge not shown (bug 2)
3. Place a capacitor with a scope → enable Show Charge → uncheck Show Charge → recheck Show Charge → graph history is gone (bug 3)

🤖 Generated with Claude Code

[pfalstad]

You already fixed the issue I noted on #248. What does this fix?