feat: circuit builder, electronics lab, SPICE simulator, expanded skill tree

Workbenches:
- Circuit Builder: drag-and-drop logic gates, wire connections, truth table verification, fullscreen mode
- Electronics Lab: SPICE-like DC simulator with MNA solver, voltage sources, resistors, capacitors, LEDs, switches, NMOS/PMOS transistors, voltmeter, ammeter, play/pause simulation, fullscreen mode
- Explanation renderer: auto-detects ASCII truth tables and renders them as styled HTML

Skill tree:
- 65+ nodes across 19 groups spanning math → electronics → CPU → ASM → OS → networking → web
- Groups: Aritmética, Álgebra, Lógica, Electrónica, Circuitos Digitales, Secuenciales, Tu CPU, Verilog/HDL, Arquitectura Extendida, Sistemas Operativos, Programación en C, Redes, La Web, Señales, Síntesis Audio, Gráficos, Tu Consola
- Dependency highlighting: clicking a node dims all others and highlights the full path
- Group boxes with colored borders around related nodes
- Dependency chain audit: fixed illogical prerequisites throughout the tree

Content:
- 24 electronics challenges (basics, series/parallel, capacitors, diodes, transistors, op-amps, power supplies)
- 12 circuit builder challenges (logic gates, NAND universality, combinational circuits)
- Fixed all explanation spoilers: examples now use different numbers than the challenge questions
- Probe system now requires voltmeter/ammeter instruments instead of checking arbitrary node IDs

UX:
- Custom dark-themed scrollbars
- Fullscreen mode for circuit/electronics editors (portal-based, Esc to exit)
- SVG coordinate fix using getScreenCTM for accurate wire placement in fullscreen
- Meter reading labels positioned correctly regardless of component rotation
- Scratchpad defaults to closed, persists open/close state in localStorage
- Empty placeholder nodes show "Próximamente" instead of appearing completed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/stores/useProgressStore.ts

@@ -126,7 +126,7 @@ export const useProgressStore = create<ProgressStore>()(
         const state = get();
         const completedNodeIds: string[] = [];
         for (const node of skillNodes) {
-          const allDone = node.challenges.every(
+          const allDone = node.challenges.length > 0 && node.challenges.every(
             (cId) => state.completedChallenges[`${node.id}/${cId}`]
           );
           if (allDone) completedNodeIds.push(node.id);
@@ -141,7 +141,7 @@ export const useProgressStore = create<ProgressStore>()(
 
         const completedNodeIds = get().getCompletedNodeIds();
 
-        const allDone = node.challenges.every(
+        const allDone = node.challenges.length > 0 && node.challenges.every(
           (cId) => state.completedChallenges[`${node.id}/${cId}`]
         );
         if (allDone) return 'completed';