fix(ui): consistent hashrate units, full-address tooltips, drop dead vars

- Balance card hashrate now uses the shared FormatHashrate() (TH/GH/MH/KH/H)
  instead of a bespoke two-tier KH/s formatter.
- Recent-tx rows show the full untruncated address on hover — two z-addresses can
  truncate to the same first/last window — and the truncate helpers guard maxLen<=3.
- Remove the unused viewTop/viewBot "viewport culling" locals in the tx list
  (pagination already bounds per-frame work).

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

src/ui/windows/balance_tab_helpers.cpp

@@ -46,7 +46,7 @@ std::string timeAgo(int64_t timestamp)
 
 std::string truncateAddress(const std::string& address, int maxLen)
 {
-    if (address.length() <= static_cast<size_t>(maxLen)) return address;
+    if (maxLen <= 3 || address.length() <= static_cast<size_t>(maxLen)) return address;
     int half = (maxLen - 3) / 2;
     return address.substr(0, half) + "..." + address.substr(address.length() - half);
 }