feat(seed): glow the Migrate-to-seed button for a legacy wallet

A pre-seed-phrase (legacy, non-mnemonic) wallet is exactly the one that benefits
from migrating — so nudge the user by glowing the Settings "Migrate to seed"
button with a soft pulsing accent halo.

Adds a cached wallet mnemonic status (probeWalletSeedStatus), classified once per
connect via z_exportmnemonic (the same signal the migration Intro pre-flight
uses): NoMnemonic = legacy → glow; HasMnemonic / Incapable (old daemon) / while
locked or on lite = no glow. Reset on disconnect so it re-probes after a wallet
switch or a post-migration adopt, with a small attempt cap to avoid re-probing on
a persistent transient error. The sweep forces the status so the glow is
captured (restored after). Verified dark + light.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app_sweep.cpp

@@ -113,6 +113,8 @@ void App::installDemoWalletData()
     applyHealthyDemoState();
     state_.sync.blocks = state_.sync.headers = 3124322;
     state_.sync.verification_progress = 1.0; state_.sync.syncing = false;
+    // Legacy (pre-seed) wallet so the Settings "Migrate to seed" button glows in the sweep.
+    wallet_seed_status_ = WalletSeedStatus::NoMnemonic;
 
     state_.privateBalance = 12.50000000; state_.transparentBalance = 3.25000000;
     state_.totalBalance = 15.75000000; state_.unconfirmedBalance = 0.50000000;
@@ -210,6 +212,7 @@ void App::clearDemoWalletData()
 {
     auto& s = sweep_state_snapshot_;
     if (!s.valid) return;
+    wallet_seed_status_ = WalletSeedStatus::Unknown;   // re-probe on the next real connect
     state_.connected = s.connected; state_.warming_up = s.warming_up;
     state_.daemon_initializing = s.daemon_initializing;
     state_.encrypted = s.encrypted; state_.locked = s.locked;