feat: cast-while-mounted dismounts then casts (retail behavior)

Previously pressing any spell while mounted just dismounted without
casting. Match retail: dismount the (ground) mount and send the cast in
the same action. While airborne on a flying mount, block the cast with
'You can't do that while flying' instead of dropping the player out of
the sky. startCraftQueue guards the flying case too so the craft queue
isn't left populated with no cast in flight.

src/game/spell_handler.cpp

@@ -635,10 +635,18 @@ void SpellHandler::castSpell(uint32_t spellId, uint64_t targetGuid) {
         return;
     }
 
-    // Casting any spell while mounted → dismount instead
+    // Casting any spell while mounted → dismount first, then cast (retail
+    // behavior: the mount is a cancellable aura that a cast removes). Exception:
+    // while actively flying (airborne on a flying mount) retail blocks the cast
+    // entirely rather than dropping the player out of the sky, so bail there.
+    // dismount() clears the local mount state synchronously, so control falls
+    // through and the cast is sent from the ground in the same action.
     if (owner_.isMounted()) {
+        if (owner_.isPlayerFlying()) {
+            owner_.addUIError("You can't do that while flying.");
+            return;
+        }
         owner_.dismount();
-        return;
     }
 
     if (casting_) {
@@ -848,14 +856,15 @@ void SpellHandler::cancelCast() {
 }
 
 void SpellHandler::startCraftQueue(uint32_t spellId, int count) {
-    // Crafting while mounted must dismount first, then craft — matching retail.
-    // castSpell() bails out early when mounted (dismount-instead-of-cast), so
-    // dismount here BEFORE populating the queue. dismount() clears the local
-    // mount state synchronously, so the castSpell() below sees isMounted()==false
-    // and actually sends the cast. Without this, the queue was left populated
-    // with no cast in flight, freezing the crafting UI on "Crafting... N
-    // remaining" until the player manually mounted and dismounted again.
-    if (owner_.isMounted()) owner_.dismount();
+    // castSpell() dismounts a ground mount and then casts, so crafting while
+    // mounted just works. But while airborne on a flying mount it blocks the
+    // cast — guard that here too so we don't populate the queue with a cast
+    // that will never fire (which would freeze the crafting UI on
+    // "Crafting... N remaining").
+    if (owner_.isMounted() && owner_.isPlayerFlying()) {
+        owner_.addUIError("You can't do that while flying.");
+        return;
+    }
     craftQueueSpellId_ = spellId;
     craftQueueRemaining_ = count;
     castSpell(spellId, 0);