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/app.h

@@ -811,7 +811,7 @@ private:
     bool import_in_progress_ = false;     // an import + rescan is running (disable/spinner)
     std::string import_result_address_;   // address imported on success (shown as a copy field)
     bool import_view_mode_ = false;       // dialog mode: false = spending key, true = viewing key
-    char import_key_scan_height_[16] = {0};  // optional rescan start height (viewing-key mode)
+    char import_key_scan_height_[16] = {0};  // optional rescan start height (shielded spend / viewing-key imports)
     std::string backup_status_;
     bool backup_success_ = false;