Remove impossible mechanics and fix game balance

- Remove lifesteal from spells (player has no health to heal)
- Remove execute effects (too powerful instant-kill mechanic)
- Remove freeze status effect (doesn't fit game design)
- Remove knowledgeRetention skill (study progress is now always saved)
- Fix soulBinding skill (now binds guardian essence to equipment)
- Buff ancientEcho skill (+1 capacity per level instead of per 2 levels)
- Rename lifesteal_5 to mana_siphon_5 in enchantment effects
- Update guardian perks:
  - Water: 10% double cast chance instead of lifesteal
  - Dark: 25% crit chance instead of lifesteal
  - Life: 30% damage restored as mana instead of healing
  - Death: +50% damage to enemies below 50% HP instead of execute
- Add floor armor system (flat damage reduction)
- Update spell effects display in UI
- Fix study cancellation - progress is always saved when pausing

src/lib/game/store/combatSlice.ts

@@ -116,13 +116,6 @@ export const createCombatSlice = (
         log.unshift('✨ Spell Echo! Double damage!');
       }
 
-      // Lifesteal effect
-      const lifestealEffect = spellDef.effects?.find(e => e.type === 'lifesteal');
-      if (lifestealEffect) {
-        const healAmount = dmg * lifestealEffect.value;
-        rawMana = Math.min(rawMana + healAmount, maxMana);
-      }
-
       // Apply damage
       floorHP = Math.max(0, floorHP - dmg);
       castProgress -= 1;