feat: discipline UI improvements - stat labels, prerequisites, mana type tab

- Add player-friendly label field to statBonus in DisciplineDefinition
- Show prerequisite requirements on locked discipline cards
- Disable activate button for locked disciplines
- Restructure elemental attunement into dedicated 'Mana Types' tab
- Add checkDisciplinePrerequisites utility function
- Update store to enforce prerequisite checking on activation
- Split discipline-prerequisites tests into separate file

src/lib/game/data/disciplines/invoker.ts

@@ -12,7 +12,7 @@ export const invokerDisciplines: DisciplineDefinition[] = [
     manaType: 'light',
     baseCost: 10,
     description: 'Improve spell power and effectiveness.',
-    statBonus: { stat: 'baseDamageBonus', baseValue: 6 },
+    statBonus: { stat: 'baseDamageBonus', baseValue: 6, label: 'Base Damage' },
     difficultyFactor: 130,
     scalingFactor: 65,
     drainBase: 3,
@@ -40,7 +40,7 @@ export const invokerDisciplines: DisciplineDefinition[] = [
     manaType: 'void',
     baseCost: 15,
     description: 'Master the exotic void mana for devastating effects.',
-    statBonus: { stat: 'baseDamageMultiplier', baseValue: 0.15 },
+    statBonus: { stat: 'baseDamageMultiplier', baseValue: 0.15, label: 'Base Damage Multiplier' },
     difficultyFactor: 200,
     scalingFactor: 100,
     drainBase: 7,