fix: update somark.tech to somark.cn/somark.ai with purchase URLs (#16892)

## Summary

Update all `somark.tech` references to `somark.cn` (default for China) /
`somark.ai` (for overseas including Taiwan, China; Hong Kong, China;
Macau, China). Users fill in `base_url` manually — default is
`somark.cn`.

### Changes

| File | Change |
|------|--------|
| `constants.py` | Default → `somark.cn` |
| `somark_parser.py` | SAAS_BASE_URL + fallback → `somark.cn` |
| `ocr_model.py` | Default → `somark.cn` |
| `pdf_parser_common.go` | Go default → `somark.cn` |
| `llm.ts` | API key URL → `somark.cn` |
| `en.ts` | Base URL descriptions + purchase URLs |
| `zh.ts` | Base URL descriptions + purchase URLs |

### Purchase URLs
- China: `https://somark.cn/workbench/purchase`
- Overseas: `https://somark.ai/studio/purchase`

---------

Co-authored-by: justinychuang <huangyicheng@soulcode.cn>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

internal/parser/parser/pdf_parser_common.go

@@ -109,7 +109,7 @@ func NewPDFParser() *PDFParser {
 		MinerUPollTimeout:              minerUPollTimeout,
 		PaddleOCRAlgorithm:             "PaddleOCR-VL",
 		OpenDataLoaderTimeout:          600,
-		SoMarkBaseURL:                  "https://somark.tech/api/v1",
+		SoMarkBaseURL:                  "https://somark.cn/api/v1",
 		SoMarkImageFormat:              "url",
 		SoMarkFormulaFormat:            "latex",
 		SoMarkTableFormat:              "html",