Wallet: cap deposit / invoice amount with sensible max

[dmnyc]

Problem

The Receive screen (WalletScreen.kt → ReceiveAmountContent) currently accepts any user-entered amount with no client-side cap. If the user enters an amount that exceeds:

• Spark: per-channel inbound liquidity / JIT channel limit
• NWC: the backing wallet's max receive policy

…the invoice creation fails with a backend error string surfaced via _sendError, which is bad UX. The user only learns the cap exists by hitting it.

Investigation done

• breez_sdk_spark 0.11.0 does not expose a queryable max-receive method. FetchConversionLimits only returns minFromAmount / minToAmount (for fiat onramps, not Lightning receive).
• NIP-47 / our NwcRepository get_info response (alias, methods, pubkey, blockHeight) has no budget or max-amount fields. NIP-47 has no widely-implemented get_budget method.

Proposal

Pick a sensible static cap as a starting point (e.g. 1,000,000 sats / 0.01 BTC) applied to both Spark and NWC, with the field showing a supporting line like "Max 1,000,000 sats" and the Create invoice button disabled when exceeded.

This cap should match across Android and iOS — see companion issue on barrydeen/wisp-ios for iOS-side work.

Open questions

1. What's the right number? 1M sats is ~$1000 at current price — generous for everyday Lightning use, well under typical channel limits.
2. Should the cap differ per wallet backend? (Spark vs. NWC). Probably not for v1 — pick one number for simplicity.
3. Should we tighten this dynamically once Spark SDK or a future NIP-47 extension exposes real limits? File-as-followup.

Related

• iOS companion: TBD link once created
• Touches Android feat(wallet-setup): primary Spark mode + More-options accordion #570–fix(settings): isolate instant-zap defaults per account #576 slim-PR series (Receive screen redesign, all-prs branch)

[dmnyc]

iOS companion filed: barrydeen/wisp-ios#209 — same cap should apply on both platforms.