Multi-account support and streaming fixes

- Allow multiple linked accounts per service (YouTube, Twitch)
- LinkedAccount PK changed from serviceId to backend UUID
- StreamDestination now references linkedAccountId
- Room DB migration v2→v3 for new schema
- Unlink endpoint changed to DELETE by account ID
- Accounts UI always shows "Add Account" for all providers
- preparePlan matches destinations by ID instead of serviceId

shared/src/main/java/com/omixlab/lckcontrol/shared/LinkedAccount.kt

@@ -4,6 +4,7 @@ import android.os.Parcel
 import android.os.Parcelable
 
 data class LinkedAccount(
+    val id: String,
     val serviceId: String,
     val displayName: String,
     val accountId: String,
@@ -12,6 +13,7 @@ data class LinkedAccount(
 ) : Parcelable {
 
     constructor(parcel: Parcel) : this(
+        id = parcel.readString()!!,
         serviceId = parcel.readString()!!,
         displayName = parcel.readString()!!,
         accountId = parcel.readString()!!,
@@ -20,6 +22,7 @@ data class LinkedAccount(
     )
 
     override fun writeToParcel(parcel: Parcel, flags: Int) {
+        parcel.writeString(id)
         parcel.writeString(serviceId)
         parcel.writeString(displayName)
         parcel.writeString(accountId)