Splitting checks and adding items mid-meal (guest flow)

When a guest scans the table QR, they don't just see a static bill — they can split, tip, add more items, and pay their portion without flagging down a server.

Splitting evenly:
1. On the bill view, the guest taps "Split evenly"
2. They pick the number of guests at the table (the "Number of guests" stepper)
3. They pick which share they're paying (the "I'm paying for" stepper — usually 1 of N)
4. The "Your share" line updates live — that's what they'll pay, including their proportional slice of the tip
5. They tap Apple Pay / Google Pay / Card and only their share is charged

What happens behind the scenes:
Each share creates its own Stripe payment intent. The check stays open until the cumulative paid amount covers the order total. When the last sharer settles up, the check flips to Paid automatically and the table goes Available — no server intervention needed.

Live "Already paid by others" indicator:
Once at least one sharer has paid, every other phone at the table that's still on the bill view shows:
• "Already paid by others: $XX.XX"
• "Remaining due on this check: $YY.YY"

So sharer #3 walks up after sharers #1 and #2 paid, scans the QR, and sees exactly what's left.

Adding items mid-meal:
1. While the check is still open, the bill view shows a dashed "+ Add another item" button below the order items
2. Tapping it opens the menu with the current check pre-selected as the join target
3. The guest adds items, places the order — they merge straight into the open check and the kitchen sees them on the KDS
4. After placing, a "Back to Check & Pay" button bounces the guest back to the bill view

Splitting unevenly (one person ordered the steak, the other the salad):
For v1, the guest-side splitter is even-only. If a party wants to split unevenly:
• They can each pay a "custom amount" approximating their share by picking a fractional shareOf (e.g. shareOf = 4 and one person pays 2-of-4 = half)
• Or the server can split the check on the admin side (Floor Plan → table → Split Check → "By items") and hand each guest a fresh /restaurant/pay link

Plain card with split:
The inline Stripe card form supports splits too — the patron's card is charged just for their share. No redirect to a hosted page; everything happens in their browser tab.

What's NOT in the split:
• The Google review prompt only appears at the very end (after the last sharer pays the check off and gives a positive meal rating)
• Each sharer rates their own meal independently; the review nudge surfaces for whichever sharer closes the bill
• Loyalty points + room-charge still flow through the full-check flow today; in a split, only the last sharer can hit "room charge" cleanly