refactor: increased fixed width constraint from image asset helper

[abdallahshaban557]

Description of what this PR is changing or adding, and why:

Update image resolution based on feedback from customers using larger pixel density screens.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

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[gemini-code-assist]

Code Review

This pull request updates the default asset width from 800 to 1200 in the Image component. There are no review comments, and I have no feedback to provide.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 05962bf):

https://flutter-docs-prod--docs-pr13552-fix-image-resolution-bq5cgwa2.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit bce7787):

https://flutter-dev-230821--www-pr13552-fix-image-resolution-ggf389s1.web.app