feat(ui): migrate Explorer detail/loading + daemon-update prompt to reference design

Presentation-only reference-design migration:
- Explorer loading modal -> struct-form overlay (idSuffix explorerloading); block-
  detail modal loses its header divider; the search button + the prev/next nav
  become TactileButton; TR() the hardcoded "Previous block"/"Next block" tooltips,
  "TxID:", and the "shielded" output fallback (3 new explorer_* keys + reuse the
  existing `shielded` key), all 8 langs. Block-fetch RPC path untouched.
- Daemon-update startup prompt -> struct-form overlay (idSuffix daemonupdprompt),
  2 StyledButton -> TactileButton, drop the footer divider. The reinstall/
  prompted-size persistence logic is unchanged.

Build + ctest + hygiene clean; "DRGX" ticker left hardcoded per convention.

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

src/util/i18n.cpp

@@ -1937,6 +1937,9 @@ void I18n::loadBuiltinEnglish()
         strings_["explorer_block_merkle"] = "Merkle Root";
         strings_["explorer_tx_outputs"] = "Outputs";
         strings_["explorer_tx_size"] = "Size";
+        strings_["explorer_txid"] = "TxID:";
+        strings_["explorer_prev_block"] = "Previous block";
+        strings_["explorer_next_block"] = "Next block";
         strings_["explorer_invalid_query"] = "Enter a block height or 64-character hash";
         strings_["explorer_invalid_response"] = "Invalid response from daemon";
         strings_["explorer_hash_not_found"] = "No block or transaction found for this hash";