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.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -62,7 +62,7 @@ body:
         - Video tools
         - Audio tools
         - PDF tools
-        - Data tools
+        - File tools
         - Image editor
         - Pipelines / automation
         - File library