Add Bulgarian language support (#13147)

### What problem does this PR solve?

RAGFlow supports 12 UI languages but does not include Bulgarian. This PR
adds Bulgarian (`bg` / `Български`) as the 13th supported language,
covering the full UI translation (2001 keys across all 26 sections) and
OCR/PDF parser language mapping.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### Changes

- **`web/src/constants/common.ts`** — Registered Bulgarian in all 5
language data structures (`LanguageList`, `LanguageMap`,
`LanguageAbbreviation` enum, `LanguageAbbreviationMap`,
`LanguageTranslationMap`)
- **`web/src/locales/config.ts`** — Added lazy-loading dynamic import
for the `bg` locale
- **`web/src/locales/bg.ts`** *(new)* — Full Bulgarian translation file
with all 26 sections, matching the English source (`en.ts`). All
interpolation placeholders, HTML tags, and technical terms are preserved
as-is
- **`deepdoc/parser/mineru_parser.py`** — Mapped `'Bulgarian'` to
`'cyrillic'` in `LANGUAGE_TO_MINERU_MAP` for OCR/PDF parser support

### How it works

The language selector automatically picks up the new entry. When a user
selects "Български", the translation bundle is lazy-loaded on demand.
The preference is persisted to the database and localStorage across
sessions.

deepdoc/parser/mineru_parser.py

@@ -73,6 +73,7 @@ LANGUAGE_TO_MINERU_MAP = {
     'Thai': 'th',
     'Greek': 'el',
     'Hindi': 'devanagari',
+    'Bulgarian': 'cyrillic',
 }