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/util/i18n.cpp

@@ -474,6 +474,8 @@ void I18n::loadBuiltinEnglish()
         strings_["mig_receive_addr"] = "New wallet receive address:";
         strings_["mig_copy_seed"] = "Copy seed";
         strings_["mig_backed_up"] = "I've written down my seed phrase";
+        strings_["mig_understood"] = "I understand that once I continue, my funds will be moved into this new wallet and can only be recovered with this seed phrase";
+        strings_["mig_ack_required"] = "Confirm both statements above to continue";
         strings_["mig_step1_done"] = "Step 1 of 2 complete — your funds have NOT moved yet. Sweeping them into this new wallet is the next step.";
         strings_["mig_continue_sweep"] = "Continue to sweep";
         strings_["mig_discard"] = "Discard migration";
@@ -491,6 +493,10 @@ void I18n::loadBuiltinEnglish()
         strings_["mig_adopt"] = "Make this my wallet";
         strings_["mig_remainder"] = "Some funds are still in your old wallet (too many inputs for one transaction) — sweep the remainder before switching.";
         strings_["mig_sweep_remaining"] = "Sweep remaining";
+        strings_["mig_discard_confirm"] = "Discard this migration? The new seed wallet created on this device will be deleted. Make sure you have not sent anything to it. Your current wallet is not changed.";
+        strings_["mig_discard_yes"] = "Yes, discard";
+        strings_["mig_sweep_confirm"] = "Send %.8f DRGX from your current wallet to the new seed wallet in a single transaction? This moves all of your funds.";
+        strings_["mig_sweep_confirm_yes"] = "Yes, sweep funds";
         strings_["mig_waiting_mine"] = "Waiting for the sweep transaction to be mined…";
         strings_["mig_adopting"] = "Installing your new wallet and rescanning…";
         strings_["mig_done"] = "Migration complete. Your wallet is now backed by your seed phrase.";