feat(balance): show a "catching up — funds are safe" strip while syncing

A syncing node skips the balance RPC, so the balance card paints a bare
0.00000000 / $-.-- that reads as lost funds after an update (a documented
support trap). Add one shared notice at the top of the Balance tab — above
every layout — shown only for the normal catching-up case (connected, network
tip known, still verifying, not a rescan/witness rebuild). Offline, no-peers,
a busy node and rescans stay the node-status banner's job, so the two never
stack.

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

src/util/i18n.cpp

@@ -1237,6 +1237,7 @@ void I18n::loadBuiltinEnglish()
         strings_["balance_shielded_fmt"] = "Shielded: %.8f";
         strings_["total_balance_label"] = "Total Balance";
         strings_["balance_syncing_pct"] = "Syncing %.1f%%";
+        strings_["balance_catchup_notice"] = "Catching up to the network (%.0f%%) — your balance will finish updating when sync completes. Your funds are safe.";
         strings_["balance_mining_rate"] = "Mining %s";
         strings_["balance_layout_switched"] = "Layout: %s";
         strings_["byte_count_fmt"] = "%zu / %zu bytes";