feat(settings): rename to "Import Private Key" + add scan height to it

Follow-up to the key/viewing-key button split:

- Rename the "Import Key…" button to "Import Private Key…" (settings_import_key
  value in en + all 8 langs, reusing the existing import_key_title wording) so it
  reads as the natural counterpart to "Import Viewing Key…". The dialog title was
  already "Import Private Key". The viewing-key wrong-type redirect hint now names
  the renamed button.

- Show the "scan from block height" field in the private-key (spending) dialog too,
  not just the viewing-key one. z_importkey accepts a start height, so a shielded
  spend import of a recent key needn't rescan the whole chain. Transparent WIF
  (importprivkey) has no such param, so once a transparent key is recognized the
  field is disabled, its buffer cleared, and a "Transparent keys always rescan
  fully" note replaces the hint. importPrivateKey already gates the start-height
  append on (viewing || shielded), so transparent imports never receive it.

- i18n: shared scan keys renamed import_viewkey_scan_{label,hint} ->
  import_scan_{label,hint}; added import_scan_transparent (en + 8 langs).

Reviewed via an adversarial multi-agent pass (rpc-routing / i18n / ui-logic /
consistency); only two nits surfaced (stale disabled-field value + a stale
comment), both fixed here.

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

src/util/i18n.cpp

@@ -428,7 +428,7 @@ void I18n::loadBuiltinEnglish()
         strings_["settings_shield_mining"] = "Shield Mining...";
         strings_["settings_merge_to_address"] = "Merge to Address...";
         strings_["settings_clear_ztx"] = "Clear Z-Tx History";
-        strings_["settings_import_key"] = "Import Key...";
+        strings_["settings_import_key"] = "Import Private Key...";
         strings_["settings_import_viewkey"] = "Import Viewing Key...";
         strings_["settings_export_key"] = "Export Key...";
         strings_["settings_export_all"] = "Export All...";
@@ -1365,9 +1365,10 @@ void I18n::loadBuiltinEnglish()
         strings_["import_viewkey_note"] = "Watch-only: a viewing key reveals an address's balance and transactions but cannot spend its funds.";
         strings_["import_viewkey_field"] = "Viewing key";
         strings_["import_key_wrong_type"] = "This looks like a viewing key. Use \"Import Viewing Key\" instead.";
-        strings_["import_viewkey_wrong_type"] = "This looks like a spending key. Use \"Import Key\" instead.";
-        strings_["import_viewkey_scan_label"] = "Scan from block height (optional)";
-        strings_["import_viewkey_scan_hint"] = "0 = rescan from the start";
+        strings_["import_viewkey_wrong_type"] = "This looks like a spending key. Use \"Import Private Key\" instead.";
+        strings_["import_scan_label"] = "Scan from block height (optional)";
+        strings_["import_scan_hint"] = "0 = rescan from the start";
+        strings_["import_scan_transparent"] = "Transparent keys always rescan fully";
 
         // --- Key Export Dialog ---
         strings_["key_export_fetching"] = "Fetching key from wallet...";