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.cpp

@@ -742,6 +742,10 @@ void App::update()
     // One-time reminder to back up the wallet's seed phrase (mnemonic wallets only).
     maybeRemindSeedBackup();
 
+    // Classify the wallet's mnemonic status (once per connect) so the Migrate-to-seed button can
+    // glow for a legacy, pre-seed-phrase wallet.
+    probeWalletSeedStatus();
+
     // Pick up progress/result from a running seed-wallet migration (create/sweep/adopt).
     pumpSeedMigration();
     // While confirming the sweep, poll the tx confirmations + legacy balance every ~5s.