A switch for sliding sync, and a warning under it

The window half of #190. A toggle in the Matrix account tools, off unless
somebody turns it on, saying what it does and what to do if it goes wrong -
"if this account stops receiving messages, turn it back off" - because that
is the failure mode and it should not have to be guessed at.

Reconnecting the account is what picks the change up, since which sync to
speak is decided once per connection, and the toast says so rather than
leaving somebody watching an unchanged window.

Closes #190.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/renderer/src/components/MatrixAccountTools.tsx

@@ -115,6 +115,34 @@ export function MatrixAccountTools({ account }: { account: Account }): JSX.Eleme
 
   return (
     <div className="matrix-tools">
+      <div className="setting-row">
+        <div className="setting-text">
+          <div>Sliding sync</div>
+          <div className="small muted">
+            A newer, much lighter way of talking to the homeserver: it sends the rooms that
+            changed instead of the whole account every time. Worth it on an account in a lot of
+            rooms. Still new here — if this account stops receiving messages, turn it back off.
+          </div>
+        </div>
+        <button
+          type="button"
+          className={account.slidingSync ? 'button' : 'button subtle'}
+          onClick={() =>
+            void rpc('setMatrixSlidingSync', { accountId: account.id, enabled: !account.slidingSync })
+              .then(() =>
+                store.toast(
+                  'info',
+                  account.slidingSync
+                    ? 'Sliding sync off — reconnect this account to go back to the old sync'
+                    : 'Sliding sync on — reconnect this account to start using it'
+                )
+              )
+              .then(() => void store.refreshAccounts())
+          }
+        >
+          {account.slidingSync ? 'On' : 'Off'}
+        </button>
+      </div>
       <div className="setting-row">
         <div className="setting-text">
           <div>Call server</div>

src/shared/wire.ts

@@ -55,6 +55,12 @@ export interface Account {
    * homeservers do name one.
    */
   rtcFocusUrl?: string
+  /**
+   * Matrix only: whether this account uses sliding sync where the homeserver
+   * offers it. Off unless somebody turned it on - it replaces the whole of how
+   * the account talks to its server, so it is not taken automatically.
+   */
+  slidingSync?: boolean
 }
 
 export interface SneedChatRoom {