Fix JPG format and base64 preview on web

[gre]

Summary

• Fix JPG captures silently producing PNG on web: toDataURL("image/jpg") is not a valid MIME type, browsers require "image/jpeg". Now maps "jpg" → "jpeg" before calling toDataURL.
• Fix base64 result appearing blank in example: raw base64 was used directly as <Image source={{uri}}>, which doesn't work without the data:image/...;base64, prefix.

Ref #586

Test plan

• npm run type-check / npm run lint:ci pass
• Open example-web, test all 4 capture buttons on BasicTestScreen:
  • PNG (Data URI) — works as before
  • JPG (Data URI) — now produces actual JPEG
  • PNG (Base64) — now displays correctly (was blank)
  • JPG (Base64) — now displays correctly (was blank)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes two web-specific capture/preview issues in react-native-view-shot and its web example: ensuring JPG captures are encoded as real JPEGs (not silently PNG) and ensuring base64 capture results display correctly in the example app.

Changes:

• Map format: "jpg" to the correct canvas MIME type (image/jpeg) before calling toDataURL on web.
• Prefix raw base64 capture output with a proper data:image/...;base64, header in example-web so <Image source={{uri}}> renders it.
• Minor refactor to compute MIME type once before building the data URL / data URI.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/RNViewShot.web.ts	Ensures correct MIME type (image/jpeg) is used for JPG on web canvas exports.
example-web/src/screens/BasicTestScreen.tsx	Fixes example preview for base64 results by converting raw base64 into a data URI.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.

[gre]

tested manually.
waiting for Test Web Example Playwright to succeed.