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>

CMakeLists.txt

@@ -484,6 +484,7 @@ set(APP_SOURCES
     src/util/base64.cpp
     src/util/single_instance.cpp
     src/util/i18n.cpp
+    src/util/text_format.cpp
     src/util/platform.cpp
     src/util/payment_uri.cpp
     src/util/texture_loader.cpp
@@ -991,6 +992,8 @@ if(BUILD_TESTING)
         src/util/payment_uri.cpp
         src/util/amount_format.cpp
         src/util/address_validation.cpp
+        src/util/i18n.cpp
+        src/util/text_format.cpp
         src/data/wallet_state.cpp
         src/data/transaction_history_cache.cpp
         src/daemon/lifecycle_adapters.cpp