Centralize config + EncryptedSharedPreferences for credentials

- New AppConfig singleton backed by EncryptedSharedPreferences
- Credentials stored in Android Keystore (AES256-GCM)
- Removed hardcoded creds from ClippySession and SetupActivity
- Access token caching to avoid re-login on each session
- Defaults seeded on first run for backward compatibility

Co-authored-by: Spirit <spirit@hobbs.farm>

app/src/main/java/farm/hobbs/clippyauto/ClippyAutoApp.kt

@@ -1,17 +1,23 @@
 package farm.hobbs.clippyauto
 
 import android.app.Application
+import farm.hobbs.clippyauto.config.AppConfig
+import farm.hobbs.clippyauto.matrix.MatrixRestClient
 
 class ClippyAutoApp : Application() {
 
-    lateinit var matrixClient: farm.hobbs.clippyauto.matrix.MatrixRestClient
+    lateinit var matrixClient: MatrixRestClient
         private set
 
     override fun onCreate() {
         super.onCreate()
         instance = this
-        matrixClient = farm.hobbs.clippyauto.matrix.MatrixRestClient(
-            homeserverUrl = "https://mess.hobbs.farm"
+
+        // Initialize encrypted config before anything else
+        AppConfig.init(this)
+
+        matrixClient = MatrixRestClient(
+            homeserverUrl = AppConfig.homeserverUrl
         )
     }