feat(i18n): 21-language pipeline, landing/docs/API wiring, landing+API translations

Shared Claude Code translation pipeline (scripts/i18n, no API key) plus Astro/VitePress/Scalar i18n wiring. Landing and API reference translated into all 20 languages; docs i18n wiring + English source anchors. The translated docs markdown (apps/docs/<locale>/**, 3,620 files) follows in a companion PR because it exceeds GitHub's per-PR CI file limit.

apps/docs/tools/files/epub-convert.md

@@ -2,23 +2,23 @@
 description: Convert an EPUB to PDF, DOCX, HTML, or Markdown.
 ---
 
-# Convert EPUB
+# Convert EPUB {#convert-epub}
 
 Convert an EPUB e-book to PDF, Word (DOCX), HTML, or Markdown. Remote resources inside the book are not fetched.
 
-## API Endpoint
+## API Endpoint {#api-endpoint}
 
 `POST /api/v1/tools/files/epub-convert`
 
 Accepts multipart form data with an EPUB file and a JSON `settings` field.
 
-## Parameters
+## Parameters {#parameters}
 
 | Parameter | Type | Required | Default | Description |
 |-----------|------|----------|---------|-------------|
 | format | string | Yes | - | Output format: `pdf`, `docx`, `html`, `md` |
 
-## Example Request
+## Example Request {#example-request}
 
 ```bash
 curl -X POST http://localhost:1349/api/v1/tools/files/epub-convert \
@@ -27,7 +27,7 @@ curl -X POST http://localhost:1349/api/v1/tools/files/epub-convert \
   -F 'settings={"format": "pdf"}'
 ```
 
-## Example Response
+## Example Response {#example-response}
 
 Returns `202 Accepted`. Track progress via SSE at `/api/v1/jobs/{jobId}/progress`.
 
@@ -38,7 +38,7 @@ Returns `202 Accepted`. Track progress via SSE at `/api/v1/jobs/{jobId}/progress
 }
 ```
 
-## Notes
+## Notes {#notes}
 
 - Accepted input format: `.epub`.
 - Remote resources embedded in the EPUB (external images, fonts) are not fetched for security.