info_density: Calculate values for vertical alignment and apply to emoji.

[karlstolley]

This PR calculates values necessary to properly fit and vertically align inline-block elements within any given line height and font size combination. Those values are then applied as an example to inline emoji, though the techniques and values here are strong candidates to be extended to other blocks appearing within the message area (e.g., todo widget checkboxes, <time> elements).

Additionally, previous fiddly top and margin values applied to emoji have been removed here, so that similarly fiddly adjustments are made unnecessary as users adjust information density values. The consequence of this is that emoji are now properly fitted to the line height, making them appear smaller than previously (though better in line with the size of surrounding text).

CZO discussion

To test this PR in dev, you'll see the default values just by running dev on this branch. The database-stored legacy font size and line-height equivalents can be made active by deselecting Dense mode under Settings > Preferences. To see the 16px/140% version, set those values in the Message-area font size (px) and Message-area line height (%). Same with any other reasonable combination of font size and line-height you might be curious to examine.

Screenshots and screen captures:

Before	After (legacy font size/line height values)

16px/140%	100px/100% (exaggerated, line-fitted)	100px/300% (exaggerated, font-size-clamped)

Showing shift-free text at 16px/140%

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[zulipbot]

Hello @zulip/server-emoji members, this pull request was labeled with the "area: emoji" label, so you may want to check it out!

[timabbott]

Test-deploying on chat.zulip.org.

[karlstolley]

Okay. I have updated this with an entirely different approach, as discussed on CZO--which discussion I'll be updating shortly with some of the things I've discovered here and want to share more widely.