perf(ui): dedupe time-ago + allocation-free case-insensitive filter (audit #1-3)

Per-frame hot paths in the immediate-mode UI were allocating needlessly:

- Address filtering in the balance tab rebuilt a std::string filter per address AND
  containsIgnoreCase() lower-cased two fresh copies per call — ~6×N allocations/frame
  on large wallets. New util::containsIgnoreCase(string_view, string_view) is
  allocation-free, and the filter is now built once outside the loop.
- Four duplicated "time ago" implementations (balance_tab_helpers, balance_recent_tx,
  send_tab, transactions_tab) are consolidated into util::formatTimeAgo (localized long
  form) + util::formatTimeAgoShort (compact "5s ago"), preserving each call site's exact
  display style. Both use snprintf, no per-row string concatenation.
- The send-tab address-suggestion scan (a walk over the whole tx list) is memoized on the
  typed text + tx count, so it no longer recomputes every frame while the user pauses.

New src/util/text_format.{h,cpp}; the two existing containsIgnoreCase/timeAgo definitions
now delegate to it. Added to both the app and test targets (test target also gains i18n.cpp,
which text_format's localized path needs).

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

src/ui/windows/balance_tab.cpp

@@ -16,6 +16,7 @@
 #include "../../config/settings.h"
 #include "../../config/version.h"
 #include "../../util/i18n.h"
+#include "../../util/text_format.h"
 #include "../theme.h"
 #include "../layout.h"
 #include "../schema/ui_schema.h"
@@ -654,10 +655,12 @@ static void RenderBalanceClassic(App* app)
         };
         std::vector<AddrRow> rows;
         rows.reserve(state.z_addresses.size() + state.t_addresses.size());
+        // The search text is constant across the loop and containsIgnoreCase is now allocation-free,
+        // so build the filter view ONCE (was a per-address std::string + two tolower-copies/frame).
+        const std::string_view addrFilter(addr_search);
         for (const auto& a : state.z_addresses) {
-            std::string filter(addr_search);
-            if (!containsIgnoreCase(a.address, filter) &&
-                !containsIgnoreCase(a.label, filter))
+            if (!util::containsIgnoreCase(a.address, addrFilter) &&
+                !util::containsIgnoreCase(a.label, addrFilter))
                 continue;
             bool isHidden = app->isAddressHidden(a.address);
             if (isHidden && !s_showHidden) continue;
@@ -667,9 +670,8 @@ static void RenderBalanceClassic(App* app)
             rows.push_back({&a, true, isHidden, isFav, app->isMiningAddress(a.address)});
         }
         for (const auto& a : state.t_addresses) {
-            std::string filter(addr_search);
-            if (!containsIgnoreCase(a.address, filter) &&
-                !containsIgnoreCase(a.label, filter))
+            if (!util::containsIgnoreCase(a.address, addrFilter) &&
+                !util::containsIgnoreCase(a.label, addrFilter))
                 continue;
             bool isHidden = app->isAddressHidden(a.address);
             if (isHidden && !s_showHidden) continue;