fix: restore variant restoration for OpenCode 1.4.0 (#871)

OpenCode 1.4.0 moved `UserMessage.variant` from the top-level to
`UserMessage.model.variant`. OpenChamber still read the legacy location,
so variant selection silently stopped being restored on sessions served
by OpenCode 1.4.0+.

Read `model.variant` first and fall back to the legacy top-level field
so both pre-1.4 and 1.4+ OpenCode servers keep working.

packages/ui/src/components/chat/MessageList.tsx

@@ -655,7 +655,10 @@ const TurnBlock: React.FC<TurnBlockProps> = ({
 
     const turnGroupingContextBase = React.useMemo(() => {
         const userCreatedAt = (turn.userMessage.info.time as { created?: number } | undefined)?.created;
-        const rawVariant = (turn.userMessage.info as { variant?: unknown } | undefined)?.variant;
+        // OpenCode 1.4.0 moved variant from top-level to model.variant on UserMessage.
+        // Prefer the new location, fall back to the legacy one for older servers.
+        const info = turn.userMessage.info as { variant?: unknown; model?: { variant?: unknown } } | undefined;
+        const rawVariant = info?.model?.variant ?? info?.variant;
         const userMessageVariant = typeof rawVariant === 'string' && rawVariant.trim().length > 0
             ? rawVariant
             : undefined;

packages/ui/src/components/chat/ModelControls.tsx

@@ -581,7 +581,7 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
     const latestLoadedUserChoice = React.useMemo(() => {
         for (let i = currentSessionMessagesFromSync.length - 1; i >= 0; i -= 1) {
             const message = currentSessionMessagesFromSync[i] as typeof currentSessionMessagesFromSync[number] & {
-                model?: { providerID?: string; modelID?: string };
+                model?: { providerID?: string; modelID?: string; variant?: string };
                 variant?: string;
                 mode?: string;
             };
@@ -598,8 +598,11 @@ export const ModelControls: React.FC<ModelControlsProps> = ({
             const agent = typeof message.agent === 'string' && message.agent.trim().length > 0
                 ? message.agent
                 : (typeof message.mode === 'string' && message.mode.trim().length > 0 ? message.mode : undefined);
-            const variant = typeof message.variant === 'string' && message.variant.trim().length > 0
-                ? message.variant
+            // OpenCode 1.4.0 moved variant from top-level to model.variant.
+            // Prefer the new location, fall back to the legacy one for older servers.
+            const variantCandidate = message.model?.variant ?? message.variant;
+            const variant = typeof variantCandidate === 'string' && variantCandidate.trim().length > 0
+                ? variantCandidate
                 : undefined;
 
             return { id: message.id, agent, providerID, modelID, variant };