feat: add round-robin routing strategy

Implements a round-robin (least recently used) account selection strategy
alongside the existing fill-first priority system. Adds a toggle in the
Profile dashboard to switch between strategies.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/app/api/settings/route.js

@@ -1,5 +1,5 @@
 import { NextResponse } from "next/server";
-import { getSettings } from "@/lib/localDb";
+import { getSettings, updateSettings } from "@/lib/localDb";
 
 export async function GET() {
   try {
@@ -10,3 +10,14 @@ export async function GET() {
     return NextResponse.json({ error: error.message }, { status: 500 });
   }
 }
+
+export async function PATCH(request) {
+  try {
+    const body = await request.json();
+    const settings = await updateSettings(body);
+    return NextResponse.json(settings);
+  } catch (error) {
+    console.log("Error updating settings:", error);
+    return NextResponse.json({ error: error.message }, { status: 500 });
+  }
+}