feat(modality): rename "file" modality label "Data" -> "Files"

The fifth user-facing group is now Image, Video, Audio, PDF, Files
(internal modality id stays "file"; section.ts "files" was already
"Files"). Updates modality.ts label + comment, all 21 i18n locales
(categories.data "Data Files"->"Files", modalities.documentsAndFiles
"PDF & Data"->"PDF & Files", dead homePage.data), landing cards/hero
search/tools filter, docs headings, and e2e modality-tab assertions
(/^Data/ -> /^Files/, which had been failing).

apps/docs/api/rest.md

@@ -314,7 +314,7 @@ All AI tools run on your hardware (CPU or NVIDIA GPU). No internet required.
 | `to-epub` | Convert to EPUB | - (accepts .docx, .md, .html, .txt) |
 | `ocr-pdf` | PDF OCR (AI) | `quality` (fast/balanced/best), `language` (auto/en/de/fr/es/zh/ja/ko), `pages` |
 
-### Data Tools
+### File Tools
 
 | Tool ID | Name | Key settings |
 |---------|------|-------------|