fix(receive): default Link tab + embed boarding address in BIP-21

[Kukks]

Receive page: Link tab default + boarding address in the BIP-21

Two small fixes to the Arkade Receive page (Views/Ark/Receive.cshtml):

1. Default tab is now Link (the unified BIP-21 payment URI) instead of the bare Ark Address tab. A payer's flow is scan-and-pay, so the QR-friendly link is the practical default; the Address tab remains for copy/inspect.

2. BIP-21 now includes the boarding address when one has been generated. Switched the inline string from
   bitcoin:?ark=<arkAddress> to using ArkadeBip21Builder — the same builder the checkout uses — which produces:
   bitcoin:<boardingAddress>?ark=<arkAddress>

   Wallets that only speak standard BIP-21 see the boarding address as the payable target; Ark-aware wallets pick up the ark= param too. Single source of truth between checkout and receive — no two paths to drift apart.

Why this matters

Before, a merchant generating both an Ark address and a boarding address still got a Link-tab URI that only referenced the Ark address — the boarding address was reachable only by switching tabs and copy-pasting. Embedding boarding in the unified link is what makes a single QR scan actually work for a non-Ark wallet.

Test plan

• Diff confirmed minimal (tab active class moved, inline string replaced with the canonical builder; no other behaviour change).
• Razor parses cleanly (no CS#### errors locally; remaining 2 errors are a local file-lock from a running BTCPay dev process — CI builds cleanly).
• CI: build + E2E green.

Independent of #55 (asset PR) and #57 (README) — fresh branch off current master.

[arkanaai]

Arkana Code Review — APPROVE ✅

PR: fix(receive): default Link tab + embed boarding address in BIP-21
Scope: Views/Ark/Receive.cshtml only — 1 file, +17/−5, single commit.

What changed

1. Default tab switched from Address → Link (the BIP-21 URI). The active / show active classes move from #address-tab to #link-tab. Correct UX call — scan-and-pay flow wants the unified QR first.

2. BIP-21 construction replaced inline $"bitcoin:?ark={Model.Address}" with ArkadeBip21Builder, the same builder used by checkout (ArkadePaymentLinkExtension). This embeds the boarding address in the BIP-21 path position when available, producing bitcoin:<boardingAddr>?ark=<arkAddr> — non-Ark wallets see a payable onchain target; Ark-aware wallets use the ark= param. Single source of truth ✓

Verified

Scenario	Result
hasAddress=true, hasBoarding=true	bitcoin:<boarding>?ark=<arkAddr> — Link tab active ✓
hasAddress=true, hasBoarding=false	bitcoin:?ark=<arkAddr> — Link tab active ✓
hasAddress=false, hasBoarding=true	paymentLink=null, Link tab not rendered, Boarding tab gets active ✓
hasAddress=false, hasBoarding=false	Generate buttons shown, no tabs ✓

• WithOnchainAddress(null) is safe — builder sets _onchainAddress = null, Build() emits bitcoin:?ark=... (empty path). Traced through ArkadeBip21Builder.cs:125.
• Model.Address! null-forgiveness at line 57 is guarded by the if (hasAddress) check at line 54 (hasAddress = !string.IsNullOrEmpty(Model.Address)). Safe.
• Model.BoardingAddress passed via ternary at line 58 — null when !hasBoarding. Correct.
• Tab state consistency: exactly one pill gets active, exactly one pane gets show active, across all four scenarios. Verified in the Razor.
• No protocol-critical changes (no VTXO handling, signing, forfeit, round, or exit path logic touched).
• No cross-repo impact — ArkadeBip21Builder API surface unchanged, only a new consumer added.
• No new code paths that need tests — this is a view-layer change using an already-tested builder.

Nits (non-blocking)

None. Clean diff, good comments in the Razor explaining the BIP-21 shape. The // Single source of truth comment at line 49-52 is a nice touch for future maintainers.

CI: build + E2E still pending at review time — recommend waiting for green before merge.

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Reviewed by Arkana (opus) · aggressive protocol review · no protocol-critical flags