Fix stale image-only copy and add 1.x-to-2.0 migration guide (#454)

* docs: add 1.x-to-2.0 migration guide and upgrade notice

Adds MIGRATING.md with backup and upgrade steps, plus a short
"coming from 1.x?" callout in README and the docs upgrade guide
pointing existing users at it.

* fix: replace stale image-only and pre-rename data copy across product

SnapOtter grew from an image-only tool into a 5-modality suite
(Image, Video, Audio, PDF, Files), but copy in several places never
caught up. Fixes:

- dropzone.defaultFormats (i18n): every non-English locale still had
  the pure pre-2.0 image-only format list; English omitted Files
  entirely. Corrected across all 21 locales.
- settings.about.appDescription (i18n): "document, and data" workflow
  copy updated to "PDF, and file" across all 21 locales.
- constants.ts: Files category's raw name was still "Data Files".
- Landing hero subtitle, JSON-LD schema, llms.txt, and 7 spots in the
  competitor-comparison pages.
- Docs: VitePress config, supported-formats, deployment, and an
  architecture.md modality-naming nit.
- OpenAPI description, root package.json description/keyword, and a
  GitHub issue template dropdown option.

DOCKERHUB.md's separate "v1.x, image tools only" pre-release notice
is left untouched since 2.0 hasn't published to Docker Hub yet.

* test: update dropzone format-hint assertion to match corrected copy

The expected string still had the stale image-only/duplicated
PDF-Documents text from before the dropzone.defaultFormats fix.

DOCKERHUB.md

@@ -2,7 +2,7 @@
 
 ![SnapOtter, a self-hosted file manipulation suite](https://raw.githubusercontent.com/snapotter-hq/SnapOtter/main/branding/social-preview.png)
 
-Open-source, self-hostable file manipulation suite. 200+ tools across image, video, audio, documents, and files, plus a layer-based image editor and local AI. Everything runs on your own hardware, so your files never leave your network.
+Open-source, self-hostable file manipulation suite. 200+ tools across image, video, audio, PDF, and files, plus a layer-based image editor and local AI. Everything runs on your own hardware, so your files never leave your network.
 
 [![License: AGPLv3](https://img.shields.io/badge/License-AGPLv3-blue)](https://github.com/snapotter-hq/SnapOtter/blob/main/LICENSE)
 [![Website](https://img.shields.io/badge/Website-snapotter.com-blue?logo=googlechrome&logoColor=white)](https://snapotter.com)