Align web payments with Hub and preserve wallet and network context

[Zazawowow]

Web payments now retain the Hub's network, active indexer and theme, and provide one consistent Invest, Fund and Subscribe flow. Completed payments use a compact, vertically centered card with transaction and recovery actions instead of a full-width green banner.

Changes:

• Match Hub navigation, surfaces, responsive payment layouts and funding-pattern cards; preserve fixed subscription amounts and require a valid offered plan.
• Offer the desktop app for payments of 0.01 BTC or more before creating a browser wallet.
• Preserve existing wallets on mismatched network links and prevent background health checks from overwriting newly selected connection settings.
• Use asynchronous browser BIP-39 derivation, reuse the signing-key cache and yield between address derivations. Copy controls show inline success/retry feedback.
• Keep initial connecting relays eligible for project discovery while excluding failed connections.

This branch is based on current main and contains only the web-payment work and its shared dependencies. Companion Hub change: block-core/angor-hub#43

Validation:

• Debug build and Release publish passed with .NET 8.0.310.
• Full Angor.Shared.Tests suite: 166 passed, zero failed or skipped. Full Angor.Sdk.Tests suite: 349 passed, 14 existing skips, zero failures.
• Published-build completion, payment-dialog/threshold/keyboard-focus/backup and plan checks passed in Chromium 153, Firefox 155 and WebKit 26.6 on Linux, at desktop 1440×900 and phone 390×844. Both themes, all payment types, reload, transaction links, deletion guard, plan handoff, fixed prices and responsive widths were exercised.
• A real Angornet Fund payment passed invoice funding, investment broadcast, independent indexer verification, reload persistence and access to the original recovery phrase. Investment transaction: b57b2d9424c75760794286b71f0f3841db865185a5550bb666e518b225bf8a8c.
• Live Hub → testnet invoice, address-copy feedback, BIP-39 compatibility and preserving an existing wallet on a mismatched network link passed.

Remaining validation limits:

• Live Lightning payment needs configured test payer credentials. Live Invest completion needs founder approval; neither is claimed as end-to-end verified. Subscribe is covered by UI and shared protocol tests, not a live payment.
• Desktop recovery/import and real-device Safari were not exercised. Linux WebKit with phone viewport is not an iPhone test.
• Existing compiler warnings and the existing HtmlSanitizer 8.0.843 NU1902 advisory remain. No mainnet funds, deployment or merge were involved.
• Isolated Chromium Release wallet preparation passed the unchanged 1.5-second event-loop target: largest measured pause 1.18 seconds. An earlier run under concurrent browser/build load measured 1.65 seconds; this is a local measurement, not a guarantee for every device.

Browser scenarios, wallet state, recovery phrases and traces are kept outside the repository and are not uploaded.