refactor(casting): remove AirPlay references, keep extensible architecture

- Remove AirPlay from CastProtocol type union (Chromecast only for now)
- Replace AirPlay TODOs with generic 'Future: Add X for other protocols' comments
- Remove PROTOCOL_COLORS export, use hardcoded Chromecast color (#F9AB00)
- Update all component headers to be protocol-agnostic
- Keep switch statements extensible for future protocol additions
- Maintain clean architecture for easy integration of new casting protocols

Architecture remains flexible for future protocols (AirPlay, DLNA, etc.)

app/(auth)/casting-player.tsx

@@ -1,6 +1,6 @@
 /**
  * Unified Casting Player Modal
- * Full-screen player for both Chromecast and AirPlay
+ * Protocol-agnostic full-screen player for all supported casting protocols
  */
 
 import { Ionicons } from "@expo/vector-icons";
@@ -35,7 +35,6 @@ import {
   shouldShowNextEpisodeCountdown,
   truncateTitle,
 } from "@/utils/casting/helpers";
-import { PROTOCOL_COLORS } from "@/utils/casting/types";
 
 export default function CastingPlayerScreen() {
   const insets = useSafeAreaInsets();
@@ -178,7 +177,7 @@ export default function CastingPlayerScreen() {
   );
 
   const protocolColor = useMemo(
-    () => (protocol ? PROTOCOL_COLORS[protocol] : "#666"),
+    () => (protocol === "chromecast" ? "#F9AB00" : "#666"), // Google yellow
     [protocol],
   );