Avoid database access on the main thread when opening the settings

`getUserProfile()` was storing the user profile (DB read, avatar Base64 encoding, DB write) on the caller
dispatcher, i.e. the main thread when called from a presenter, causing glitches in the screen transition.
- Run the whole `getUserProfile()` on the session dispatcher.
- Make `DatabaseSessionStore` main-safe by running the queries on the IO dispatcher.
- Do not refresh the profile twice from the settings screen.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/RustMatrixClient.kt

@@ -519,12 +519,14 @@ class RustMatrixClient(
         }
     }
 
-    override suspend fun getUserProfile(): Result<MatrixUser> = getProfile(sessionId)
-        .onSuccess { matrixUser ->
-            // Also update our session storage
-            _userProfile.emit(matrixUser)
-            storeUserProfile(matrixUser)
-        }
+    override suspend fun getUserProfile(): Result<MatrixUser> = withContext(sessionDispatcher) {
+        getProfile(sessionId)
+            .onSuccess { matrixUser ->
+                // Also update our session storage
+                _userProfile.emit(matrixUser)
+                storeUserProfile(matrixUser)
+            }
+    }
 
     /**
      * Store the user profile in the session storage, including a base64 encoded thumbnail of the avatar,