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/app/pages/client/ClientRoot.tsx

@@ -19,6 +19,7 @@ import React, { MouseEventHandler, ReactNode, useCallback, useEffect, useState }
 import {
   clearCacheAndReload,
   clearLoginData,
+  IDB_VERSION_CONFLICT,
   initClient,
   logoutClient,
   startClient,
@@ -201,16 +202,34 @@ export function ClientRoot({ children }: ClientRootProps) {
               <Dialog>
                 <Box direction="Column" gap="400" style={{ padding: config.space.S400 }}>
                   {loadState.status === AsyncStatus.Error && (
+                    <>
+                      {loadState.error.message === IDB_VERSION_CONFLICT ? (
+                        <>
+                          <Text>
+                            Local data is from a newer app version and cannot be read. Clear local
+                            data to continue (you will need to log in again).
+                          </Text>
+                          <Button variant="Critical" onClick={clearLoginData}>
+                            <Text as="span" size="B400">
+                              Clear local data and reload
+                            </Text>
+                          </Button>
+                        </>
+                      ) : (
                         <Text>{`Failed to load. ${loadState.error.message}`}</Text>
                       )}
+                    </>
+                  )}
                   {startState.status === AsyncStatus.Error && (
                     <Text>{`Failed to start. ${startState.error.message}`}</Text>
                   )}
+                  {loadState.error?.message !== IDB_VERSION_CONFLICT && (
                     <Button variant="Critical" onClick={mx ? () => startMatrix(mx) : loadMatrix}>
                       <Text as="span" size="B400">
                         Retry
                       </Text>
                     </Button>
+                  )}
                 </Box>
               </Dialog>
             </Box>

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'],
   });
 
+  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);