fix(app): add friendly not-found handling

packages/backend/convex/threads.ts

@@ -211,7 +211,7 @@ export const get = query({
   handler: async (ctx, { threadId }) => {
     const ownerId = await getRequiredUserId(ctx);
     const thread = await ctx.db.get(threadId);
-    if (thread?.ownerId !== ownerId) throw new ConvexError('Thread not found.');
+    if (thread?.ownerId !== ownerId) return null;
     const spoon = thread.spoonId ? await ctx.db.get(thread.spoonId) : null;
     const job = thread.latestAgentJobId
       ? await ctx.db.get(thread.latestAgentJobId)