fix: graceful recovery for IDB schema version conflict

When matrix-sdk is briefly upgraded then reverted, the local IndexedDB
schema version is higher than the SDK expects. Detect the VersionError
DOMException and show a clear 'Clear local data and reload' button
instead of a cryptic error message.

src/client/initMatrix.ts

@@ -11,6 +11,10 @@ type Session = {
   deviceId: string;
 };
 
+// Thrown when the local IndexedDB has a higher schema version than this SDK expects.
+// This happens after a downgrade (e.g. matrix-js-sdk was briefly upgraded and then reverted).
+export const IDB_VERSION_CONFLICT = 'IDB_VERSION_CONFLICT';
+
 export const initClient = async (session: Session): Promise<MatrixClient> => {
   const indexedDBStore = new IndexedDBStore({
     indexedDB: global.indexedDB,
@@ -32,7 +36,15 @@ export const initClient = async (session: Session): Promise<MatrixClient> => {
     verificationMethods: ['m.sas.v1'],
   });
 
-  await indexedDBStore.startup();
+  try {
+    await indexedDBStore.startup();
+  } catch (e) {
+    // IDB VersionError = local DB was written by a newer SDK version (schema downgrade).
+    if (e instanceof DOMException && e.name === 'VersionError') {
+      throw new Error(IDB_VERSION_CONFLICT);
+    }
+    throw e;
+  }
   await mx.initRustCrypto();
 
   mx.setMaxListeners(50);