feat(ui): rework the consolidate/merge modal

Make Merge to Address actually serve wallet-bloat consolidation and be far less
opaque. New ShieldDialog::showConsolidate() preset (used by the large-wallet
Settings banner + alert action) frames it as "Consolidate funds" and targets
shielded notes — the bloat the nudge warns about.

- Source selector: consolidate shielded notes (ANY_SAPLING), transparent
  (ANY_TADDR), or both (*) — previously hardcoded to ANY_TADDR, which never
  reduced the shielded-witness bloat. Batch limit now applies to the right side.
- Scope: on open, count spendable UTXOs + notes (listunspent / z_listunspent)
  and show "N transparent + M shielded · ~X DRGX"; warn "repeat to finish" when
  the set exceeds one batch.
- Destination auto-selects the best spendable z-address (button enabled by
  default); empty wallets get an inline "Create shielded address" (z_getnewaddress).
- Advanced disclosure hides Fee + "Max inputs per batch" (renamed from the "UTXO
  Limit" jargon) with sane defaults.
- Inline confirm step before the fund-moving call (amount + input count + dest).
- Live progress: self-polls z_getoperationstatus to show Consolidating… →
  Done/Failed, replacing the raw opid + manual "Check status" button.

All three merge entry points now use the typed showMerge()/showConsolidate()
(no stale-static leaks from direct show(MergeToAddress)). Shield-coinbase mode
keeps working. New i18n keys fall back to English.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/pages/settings_page.cpp

@@ -1248,7 +1248,7 @@ void RenderSettingsPage(App* app) {
                 ShieldDialog::show(ShieldDialog::Mode::ShieldCoinbase);
             if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("tt_shield_mining"));
             if (BTN("##wmerge", TR("settings_merge_to_address"), ICON_MD_CALL_MERGE))
-                ShieldDialog::show(ShieldDialog::Mode::MergeToAddress);
+                ShieldDialog::showMerge();
             if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("tt_merge"));
             if (BTN("##wclear", TR("settings_clear_ztx"), ICON_MD_DELETE_SWEEP))
                 s_settingsState.confirm_clear_ztx = true;
@@ -2081,7 +2081,7 @@ void RenderSettingsPage(App* app) {
                         ImGui::Dummy(ImVec2(0, Layout::spacingXs()));
                         if (material::ActionButton("##walletconsolidate", TR("wallet_size_consolidate"),
                                                    ICON_MD_CALL_MERGE, material::ActionTier::Secondary))
-                            ShieldDialog::show(ShieldDialog::Mode::MergeToAddress);
+                            ShieldDialog::showConsolidate();
                         if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("tt_merge"));
                     }