feat: add insufficient-power tint to action bar spell slots

Spell icons now render with a purple desaturated tint when the player
lacks enough mana/rage/energy/runic power to cast them. Power cost and
type are read from Spell.dbc via the spellbook's DBC cache. The spell
tooltip also shows "Not enough power" in purple when applicable.

Priority: cooldown > GCD > out-of-range > insufficient-power so states
don't conflict. Adds SpellbookScreen::getSpellPowerInfo() as a public
DBC accessor.

include/ui/spellbook_screen.hpp

@@ -58,6 +58,12 @@ public:
     /// Triggers DBC load if needed. Used by the action bar for out-of-range tinting.
     uint32_t getSpellMaxRange(uint32_t spellId, pipeline::AssetManager* assetManager);
 
+    /// Returns the power cost and type for a spell (cost=0 if unknown/free).
+    /// powerType: 0=mana, 1=rage, 2=focus, 3=energy, 6=runic power.
+    /// Triggers DBC load if needed. Used by the action bar for insufficient-power tinting.
+    void getSpellPowerInfo(uint32_t spellId, pipeline::AssetManager* assetManager,
+                           uint32_t& outCost, uint32_t& outPowerType);
+
     /// Returns a WoW spell link string if the user shift-clicked a spell, then clears it.
     std::string getAndClearPendingChatLink() {
         std::string out = std::move(pendingChatSpellLink_);