fix(frontend): fall back to blob url when FileReader is unavailable

[tink-bot]

SVG attachment preview did new FileReader() unconditionally. Some iOS environments strip FileReader (Lockdown Mode, embedded webviews), so whole promise rejected unhandled — Sentry FRONTEND-OSS-2DW, ~106 events on iOS 18 Mobile Safari. Now SVG path only take data-url route when FileReader exist, else fall back to blob url like every other mime type.

How to verify

1. Attach an SVG file to a task and open the task detail view.
2. Click the attachment to open the preview.
3. Expected: the SVG renders in the preview.
4. In devtools, run window.FileReader = undefined, reload the page, and open the SVG attachment again.
5. Expected: the SVG still renders and no unhandled rejection appears in the console.
6. Before this PR: step 5 threw ReferenceError: Can't find variable: FileReader.

[github-actions]

Preview Deployment

Preview deployments for this PR are available at:

URL	Tag	Commit
https://pr-3736.preview.vikunja.dev	ghcr.io/go-vikunja/vikunja:pr-3736	latest
https://sha-dccd87e3f36f635324c7654eac247b1f84143427.preview.vikunja.dev	ghcr.io/go-vikunja/vikunja:sha-dccd87e3f36f635324c7654eac247b1f84143427	dccd87e

The preview environment will start automatically on first visit. Subsequent pushes to this PR will update the pr-3736 image — the preview picks up the new version on restart. The per-commit URLs point to a specific version and will not change.

Run locally with Docker

docker pull ghcr.io/go-vikunja/vikunja:pr-3736
docker run -p 3456:3456 ghcr.io/go-vikunja/vikunja:pr-3736

Last updated for commit dccd87e