feat(migration): second backup acknowledgement + confirm Discard and Sweep

Migrate-to-seed is irreversible and fund-moving, so its gates are strengthened
(UI only — the sweep/adopt/wipe logic, z_mergetoaddress args, amounts and
destination are byte-identical):
- Continue now requires a second, consequence-stating acknowledgement checkbox
  (funds move and are recoverable only via the seed phrase), not just "I wrote
  it down".
- Discard, Sweep-all and Sweep-remaining each take a second confirming click
  (stating the amount being moved) instead of firing on one click; the confirm
  body runs verbatim only after confirmation, and arming never survives a
  close/reopen.

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

src/app_sweep.cpp

@@ -354,6 +354,9 @@ void App::clearDemoWalletData()
     seed_migration_precheck_ = SeedMigrationPrecheck::Pending;
     seed_migration_precheck_started_ = false;
     seed_migration_backed_up_ = false;
+    seed_migration_understood_ = false;
+    seed_migration_confirm_discard_ = false;
+    seed_migration_confirm_sweep_ = false;
     seed_migration_balance_ = 0.0;
     seed_migration_balance_loaded_ = false;
     seed_migration_nofunds_confirmed_ = false;
@@ -623,7 +626,7 @@ void App::buildSweepCatalog()
             a.seed_migration_precheck_started_ = true;
         });
         mig("modal-migrate-showseed", SeedMigrationStep::ShowSeed,
-            [](App& a) { a.seed_migration_seed_ = kDemoMnemonic; a.seed_migration_backed_up_ = false; });
+            [](App& a) { a.seed_migration_seed_ = kDemoMnemonic; a.seed_migration_backed_up_ = false; a.seed_migration_understood_ = false; });
         mig("modal-migrate-sweep", SeedMigrationStep::Sweep,
             [](App& a) { a.seed_migration_balance_ = 15.75000000; a.seed_migration_balance_loaded_ = true; });
         mig("modal-migrate-nofunds", SeedMigrationStep::Sweep,