feat(export): CSV export scope — All / current filter / date range

Finishes the transactions-export partial. The dialog now offers three scopes:
- All transactions (unchanged default; exported with no copy).
- Current History filter — the Transactions tab publishes a TxFilterSnapshot of
  the visible legs (a merged autoshield row contributes both its send+receive
  legs, so the CSV stays a per-leg record like an unfiltered export); the radio
  is disabled with an explanation when no filter is active, and a snapshot that
  predates the current tx list (revision mismatch) is refused rather than
  silently exporting everything.
- Date range (YYYY-MM-DD, inclusive end-of-day; pending/dateless rows excluded
  with a note; invalid/reversed dates blocked inline).
The live count reflects what will actually be exported. formatTransactionsCsv
and the atomic write are unchanged.

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

src/ui/windows/transactions_tab.h

@@ -5,12 +5,34 @@
 #pragma once
 
 #include <string>
+#include <vector>
+#include <cstdint>
+#include <cstddef>
 
 namespace dragonx {
 class App;
 
 namespace ui {
 
+/**
+ * @brief Snapshot of what the History tab currently shows, for "export current filter".
+ *
+ * `indices` are positions into WalletState::transactions of every leg currently visible (a merged
+ * autoshield row contributes both its send and receive legs), sorted ascending. Published only when the
+ * filtered list is (re)built, so it reflects the last state the History tab rendered. `txRevision` lets a
+ * consumer reject a snapshot that predates the current tx list.
+ */
+struct TxFilterSnapshot {
+    bool valid = false;
+    bool active = false;                  // a type filter and/or search text is applied
+    std::uint64_t txRevision = 0;
+    std::string description;              // human label of the active filter
+    std::vector<std::size_t> indices;
+};
+
+/// The latest History-tab filter snapshot (see TxFilterSnapshot).
+const TxFilterSnapshot& GetTransactionsFilterSnapshot();
+
 /**
  * @brief Render the Transactions tab
  * Shows transaction history