Use fixed height and derive width from aspect ratio for `MessageImagePreview`

[Ruhdee]

Web introduced dynamic sizing for both image previews and inline images to implement the media_preview_size. Web does this by deciding a fixed height and deriving width from it while preserving the aspect ratio. We already do this for InlineImage.

We should also do this for MessageImagePreview. This will most likely involve moving the dynamic sizing code from InlineImage to the shared _Image helper. It also makes sense to make the spacing adjustments from PR zulip/zulip#38324 (mentioned in #2206), except those specific to inline image galleries, as a part of this issue.

This issue is a prerequisite for #2177.

References:

• #mobile > Gallery image containers have fixed size
• api_docs: Improve aspect-ratio based image preview sizing recommendations. zulip#39474

[sharayuwadghule]

Hi! I'd like to work on this issue. My plan is to extract the aspect-ratio based sizing logic currently used by InlineImage into the shared _Image helper, update MessageImagePreview to use fixed-height sizing with derived width, and apply the relevant spacing adjustments from PR #38324 excluding gallery-specific changes.

[Ruhdee]

Hi Sharayu! Please refer to https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-up-issues, specifically these points:

• Can I work on an issue that’s not marked as “help wanted”?
• I’m waiting for the next round of review on my PR. Can I pick up another issue in the meantime?

This issue does not have the "help wanted" label, and your first PR #2342 has not been reviewed by the maintainers yet.