i18n(ko): add Korean (한국어) translation (#15863)

### What problem does this PR solve?

- Add `web/src/locales/ko.ts` with full Korean translation (~3100 keys)
- Register `Ko = 'ko'` in `LanguageAbbreviation` enum (`common.ts`)
- Add `[LanguageAbbreviation.Ko]: '한국어'` to `LanguageAbbreviationMap`
- Add lazy-load entry in `web/src/locales/config.ts`
- Add `korean` key to all existing locale files (`ja`, `id`, `es`,
`pt-br`, `vi`, `zh-traditional`)
- Fix duplicate enum value `FileMimeType.Mdx` (`'text/markdown'` →
`'text/mdx'`)

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Other (please describe): Korean (한국어) i18n translation + fix
duplicate FileMimeType.Mdx enum value

web/src/constants/common.ts

@@ -94,6 +94,7 @@ export enum LanguageAbbreviation {
   Bg = 'bg',
   Ar = 'ar',
   Tr = 'tr',
+  Ko = 'ko',
 }
 
 export const LanguageAbbreviationMap = {
@@ -112,6 +113,7 @@ export const LanguageAbbreviationMap = {
   [LanguageAbbreviation.Bg]: 'Български',
   [LanguageAbbreviation.Ar]: 'العربية',
   [LanguageAbbreviation.Tr]: 'Türkçe',
+  [LanguageAbbreviation.Ko]: '한국어',
 };
 
 export const LanguageTranslationMap = {
@@ -165,7 +167,7 @@ export enum FileMimeType {
   Mp4 = 'video/mp4',
   Json = 'application/json',
   Md = 'text/markdown',
-  Mdx = 'text/markdown',
+  Mdx = 'text/mdx',
 }
 
 export const Domain = 'cloud.ragflow.io';