Add deep linking for pool positions and payment requests

[Jaydbrown]

Summary

Implements "Implement deep linking for sharing pool positions and payment requests." Custom URI scheme only (nodusprotocol://) — see note on https://nodus.fi App Links below.

• nodusprotocol://pool — opens the (single) AMM pool's detail screen. This app has exactly one pool (PoolProvider always loads a single PoolStats, no multi-pool routing exists), so there's no pool ID to thread through.
• nodusprotocol://pay?to=<address>&amount=<decimal>&token=<symbol> — opens a new PaymentRequestScreen showing the request (amount, token, requester address, copy-to-clipboard). This app has no peer-to-peer send flow yet, so this deliberately doesn't pretend to fulfill the request — it's a real, complete "view a payment request" feature, not a stub for a send flow that doesn't exist.
• AppDeepLink/parseDeepLink: a sealed-class model and pure parsing function, validating the address via the existing Validation.stellarPublicKey and rejecting non-positive/non-numeric amounts. Fully unit tested.
• DeepLinkService: wraps app_links, constructed early in main() (before runApp) so it can capture a cold-start launch link, buffering every link it receives and replaying them once to the first subscriber.
• DeepLinkListener: the piece that actually reacts to links and navigates. Security-relevant design choice: it's placed as AppLockGate's child rather than wrapping it, so its subscription — and any navigation it triggers — doesn't exist in the widget tree until the app has actually unlocked. A deep link arriving while the app is locked just sits in DeepLinkService's buffer instead of being able to navigate past the biometric lock.
• Android: registered a nodusprotocol:// intent-filter. https://nodus.fi App Links would need a hosted .well-known/assetlinks.json for domain verification, which is out of scope for this repo (no server-side access).

Test plan

• parseDeepLink unit tests: scheme/host rejection, well-formed pool + payment links, payment requests rejected for missing param / invalid address / non-numeric or non-positive amount
• DeepLinkListener widget tests: pushes PaymentRequestScreen for a payment link, stays put with no events. The pool-position path isn't covered by a widget test — it reads PoolProvider, which always constructs a real network-backed PoolService with no test seam — reviewed by hand instead.
• Could not run flutter analyze / flutter test locally — same sandbox limitation as the last two PRs. Please watch CI closely.

Note on account access

This PR is from my personal fork rather than a branch on the org repo — my org access was revoked partway through this session (confirmed via gh api repos/.../permissions: push: false where it was true earlier). Everything else about the change is unaffected.