♻️ Refactor i18n: use short keys in t9n() function

[ebullient]

Changes

• Renamed translation function from t() to t9n() for clarity

• Restructured en.ts locale file with hierarchical short keys instead of full strings:

  • Settings headings: heading.*
  • Export/Import: export-*, import.*, use-snippet.*
  • Feature toggles: drop-shadow.*, collapsible.*, collapse-type.*, etc.
  • Buttons: btn.save, btn.edit, btn.delete, etc.
  • Validation errors: error.type-empty, error.icon-invalid, etc.
  • Modal strings: modal.title.*, modal.collapsible.*
  • Per-admonition settings: admonition-type.name, admonition-title.*, etc.

• Updated zh-cn.ts locale with partial translations using new key structure

• Updated all imports and function calls throughout codebase to use t9n() instead of t()

• Improved maintainability by centralizing all English strings in en.ts

Files Modified

• src/lang/helpers.ts - Function rename
• src/lang/locale/en.ts - Complete restructure with hierarchical keys
• src/lang/locale/zh-cn.ts - Updated keys (partial translation)
• src/settings.ts, src/modal/*.ts, src/util/validator.ts - Updated all t() calls to t9n()