feat(recovery): offer legacy-fund recovery when a degraded wallet can't shield its transparent funds

Phase 1 of automating legacy wallet.dat migration: a persistent, actionable banner that detects a wallet
whose transparent funds are STUCK and offers the migrate-to-seed recovery flow (showSeedMigrationDialog).

Corrected trigger (the important part): it fires ONLY when wallet_degraded_ is set — i.e. z_getnewaddress
failed with "HD seed not found", so the wallet genuinely cannot create a shielded address to move its
funds — AND there is a transparent balance to move. It is NOT driven by "the wallet has no mnemonic": a
non-mnemonic but functional wallet (one that self-healed with a retrofit HD seed on load) has
wallet_degraded_ == false, can still shield/spend, and is deliberately never flagged — matching the
existing "legacy wallets are not nagged" policy. Full-node only.

Implemented in the already-unit-tested node banner: new NodeBannerReason::LegacyFundsStuck +
NodeBannerAction::RecoverLegacyFunds, gated on a new legacy_funds_stuck input (App computes it from
wallet_degraded_ + transparentBalance). NoPeers still takes priority (can't recover before the node sees
the chain); lite never shows it. Pure decision tests added: fires when stuck, never for a functional
wallet, yields to NoPeers, suppressed on lite.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1639,6 +1639,9 @@ void I18n::loadBuiltinEnglish()
         strings_["node_banner_open_config"] = "Open config folder";
         strings_["node_banner_auth_title"] = "Wallet can't sign in to the node";
         strings_["node_banner_auth_detail"] = "The node rejected the wallet's saved login. Restart the node, or open the config folder to fix the login in DRAGONX.conf.";
+        strings_["node_banner_legacy_title"] = "Your transparent funds can't be moved yet";
+        strings_["node_banner_legacy_detail"] = "This wallet has no usable seed, so it can't create a shielded address to move its transparent funds. Recover them into a fresh seed-backed wallet.";
+        strings_["node_banner_recover"] = "Recover funds";
         // Refresh-staleness badge (W6-2) on the Total Balance card.
         strings_["data_stale_prefix"] = "Updated";
         strings_["data_stale_tooltip"] =