fix(i18n): add missing translation keys for signatures and email import

Fixes 25 failing tests across translations and eml-import test suites:

- Add 141 missing i18n keys to en/common.json for P2.1 (extended signatures
  feature) and P2.5 (email import feature). Keys cover:
  - signatures.* (signature editor, toolbar labels, per-identity settings)
  - settings.importer.* (mail import UI labels and messages)
  - settings.tabs.signatures, settings.{loading,refresh}
  - email_composer.{insert_signature,no_signature,select_signature}
  - identities.form.{signature_store_*,use_global_default}
  - calendar.{edit,delete,duplicate}, contacts.{edit,delete,send_email}
  - contacts.import.csv_* (CSV column mapping labels)
  - Other related keys for sharing, mailbox context menu, file operations

- Propagate new keys to all 23 other locale files:
  - ar, ca, cs, da, de, es, fa, fr, he, hu, it, ja, ko, lv, nl, pl, pt, ro,
    ru, sk, tr, uk, zh
  - 19 locales with real, idiomatic translations matching existing tone
  - 4 locales (da, es, fa, zh) with English placeholders pending native
    translation

- Fix eml-import test: update accept-string expectation to match actual
  EML_IMPORT_ACCEPT constant (now includes .tgz, .tar.gz, application/gzip
  support added in P2.5)

All 53 tests now pass:
  - 48 translation completeness tests
  - 5 eml-import tests

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/__tests__/eml-import.test.ts

@@ -46,6 +46,6 @@ describe('expandImportableEmails', () => {
   });
 
   it('exposes the accept string for the file picker', () => {
-    expect(EML_IMPORT_ACCEPT).toBe('.eml,.zip,message/rfc822,application/zip');
+    expect(EML_IMPORT_ACCEPT).toBe('.eml,.zip,.tgz,.tar.gz,message/rfc822,application/zip,application/gzip');
   });
 });