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/pdf/split-pdf.md

@@ -2,17 +2,17 @@
 description: Extract pages or split a PDF into parts.
 ---
 
-# Split PDF
+# Split PDF {#split-pdf}
 
 Extract a range of pages into a new PDF, or split a document into chunks of N pages.
 
-## API Endpoint
+## API Endpoint {#api-endpoint}
 
 `POST /api/v1/tools/pdf/split-pdf`
 
 Accepts multipart form data with a PDF file and a JSON `settings` field.
 
-## Parameters
+## Parameters {#parameters}
 
 | Parameter | Type | Required | Default | Description |
 |-----------|------|----------|---------|-------------|
@@ -20,7 +20,7 @@ Accepts multipart form data with a PDF file and a JSON `settings` field.
 | range | string | When mode is `range` | - | Page range in qpdf syntax, e.g. `"1-5,8,10-z"` |
 | everyN | integer | When mode is `every` | - | Split into chunks of N pages (1-500) |
 
-## Example Request
+## Example Request {#example-request}
 
 Extract specific pages:
 
@@ -40,7 +40,7 @@ curl -X POST http://localhost:1349/api/v1/tools/pdf/split-pdf \
   -F 'settings={"mode": "every", "everyN": 10}'
 ```
 
-## Example Response
+## Example Response {#example-response}
 
 ```json
 {
@@ -51,7 +51,7 @@ curl -X POST http://localhost:1349/api/v1/tools/pdf/split-pdf \
 }
 ```
 
-## Notes
+## Notes {#notes}
 
 - In `range` mode, a single PDF containing the selected pages is returned.
 - In `every` mode, the result is a ZIP archive containing the individual parts.