Card presets + custom files: file & pasted JSON import, file export; v1.9.3

Both sections (card presets, custom files) gain two import paths and a
file download alongside the existing clipboard import:
- Import from file: hidden <input type=file accept=.json> + FileReader
  feeding the unchanged dedupe/merge logic (works on plain-HTTP LAN
  origins where navigator.clipboard is unavailable)
- Paste & import: reuses the IO textarea; guard requires '[' prefix so
  status messages are never mis-imported; empty state reveals + focuses
  the field with a hint placeholder
- Export to file: Blob download (otaman-cards.json /
  otaman-custom-files.json) via shared downloadJson() helper
- Import functions take optional text arg; clipboard stays the fallback
- data-l10n on all five buttons per section; LANG_RU entries for the
  three new labels

Version 1.9.2 -> 1.9.3 everywhere; SW cache otaman-v13 -> otaman-v14

docs/api.md

@@ -54,7 +54,7 @@ Returns server version for compatibility checking.
 
 **Example response:**
 ```json
-{"version": "1.9.2"}
+{"version": "1.9.3"}
 ```
 
 ### `GET /api/status`