Add product return (คืนสินค้า) feature to MARBOHUB POS

[Copilot]

Adds a "Return" (คืนสินค้า) order lifecycle stage to the POS app — previously paid orders could only be cancelled (pre-fulfilment), with no way to handle post-payment returns.

marbohub-pos/index.html (frontend JS)

• q() return handler — mirrors the existing f() cancel function; restores stock via addStock and sets status: "return" with a returned.ts timestamp
• "↩ คืน" button — appears in the expanded detail of paid orders (with confirm dialog); cancel button remains open-only
• Filter tab — "คืนสินค้า" tab added between "จ่ายแล้ว" and "ยกเลิก" in the orders screen
• Status badge — blue "↩ คืนสินค้า" badge, distinct from green (paid) and red (cancel)
• "all" filter — excludes both cancel and return (shows active pipeline: open + paid)
• Report — returned orders excluded from revenue/profit calculations; return count shown in the orders summary card via Q = r.filter(n => n.status === "return" && n.ts >= p)

marbohub-pos/api.php (backend)

New atomic returnOrder POST endpoint (consistent with existing addOrder pattern):

• Finds order by orderId, validates status === "paid" before mutating
• Returns 409 if status mismatch, 404 if not found, descriptive 400 for missing fields
• Restores stock overrides atomically under file lock (same semantics as addOrder stock cuts — only adjusts products already present in stockOv)

• Fixes Rt #12