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_recent_tx.cpp

@@ -1,5 +1,6 @@
 #include "balance_recent_tx.h"
 #include "../../config/version.h"
+#include "../../util/text_format.h"
 
 #include <cmath>
 #include <cstdio>
@@ -27,14 +28,7 @@ std::string formatRecentAmount(const std::string& type, double amount)
 
 std::string recentTimeAgo(int64_t timestamp)
 {
-    if (timestamp <= 0) return "";
-    int64_t now = static_cast<int64_t>(std::time(nullptr));
-    int64_t diff = now - timestamp;
-    if (diff < 0) diff = 0;
-    if (diff < 60) return std::to_string(diff) + "s ago";
-    if (diff < 3600) return std::to_string(diff / 60) + "m ago";
-    if (diff < 86400) return std::to_string(diff / 3600) + "h ago";
-    return std::to_string(diff / 86400) + "d ago";
+    return util::formatTimeAgoShort(timestamp);
 }
 }