feat(lite): add "Redownload blocks" (rescan from lite server) to Settings

Add a maintenance option for the lite wallet to re-download and re-scan every
block from the lite server — useful when balances or history look wrong.

- LiteWalletController::startRescan() runs the backend `rescan` command (which
  clears the wallet's synced block cache and re-syncs from its birthday) on a
  detached thread, reusing the existing sync progress/refresh machinery: it
  resets syncDone_ so refreshModel() shows progress again and refreshes data on
  completion. No-op if no wallet is open or a scan is already running.
- scanInProgress() exposes the initial-sync-or-rescan state.
- Settings (lite, open wallet) gains a "Redownload blocks" button behind a
  confirmation modal, disabled while a scan is running. i18n strings added.

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

src/wallet/lite_wallet_controller.h

@@ -220,6 +220,15 @@ public:
     // op produces a ready wallet; safe to call once.
     void startSync();
 
+    // Re-download and re-scan every block from the lite server: runs the backend `rescan`
+    // command (which clears the wallet's synced block cache and re-syncs from its birthday
+    // height) on a detached thread, reusing the sync progress + refresh machinery. No-op if no
+    // wallet is open or a scan is already running. True if a rescan was actually started.
+    bool startRescan();
+
+    // True while the initial sync OR a rescan is actively scanning (not yet complete).
+    bool scanInProgress() const { return syncStarted_ && !(syncDone_ && syncDone_->load()); }
+
     // Generate a new address (shielded if true, else transparent) via the backend. Fast (local
     // key derivation), safe to call on the UI thread; the next refresh lists the new address.
     LiteNewAddressResult newAddress(bool shielded);