Centralise the remaining hardcoded English literals in two otherwise
fully-translated files (explorer_tab: 40 TR calls, shield_dialog: 23) so
they join the i18n system with an English source key + fallback:
- explorer_tab: the three search-error messages (invalid response, hash
not found, not-connected) now use explorer_* keys. Also guards a hash
lookup behind an active daemon connection and disables the block-detail
prev/next nav while a fetch is in flight (both were the reason these
error paths could fire).
- shield_dialog: the operation status/error strings (submitting, submitted,
failed, status, error-checking-status, shield/merge failed) now use
shield_* keys.
- i18n: add the new explorer_* and shield_* English source keys.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
DanS
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