compose: Redesign limit indicator to show remaining characters count.

[N-Shar-ma]

compose: Remove the message_too_long banner.

Since the banner only repeated what the disabled Send button's and the limit indicator's tooltips already said, it was redundant and has been removed.

compose: Redesign limit indicator to show remaining characters count.

Additionally, the text colors have been updated for both light and dark themes, it starts showing when 900 or less characters are left, as 999 was too soon, and has a tooltip to show the maximum characters limit.

Fixes: #28706.

Screenshots and screen captures:

Light theme:
Close to limit:

Over limit:

Dark theme:
Close to limit:

Over limit:

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/design members, this pull request was labeled with the "redesign" label, so you may want to check it out!

[timabbott]

This lgtm implementation-wise other than the detail there; the one thing that maybe could be optimized further whether there's some tweak to positioning that would make it more obvious what this is. I think it's probably fine but @terpimost @alya FYI in case you want to suggest any changes before we merge this.

[alya]

Looks good to me in manual testing!

[alya]

By the way, this reminded me of #25271. Not a high priority, but I dunno if it's easy to do while this code is fresh in mind. We don't have an equivalent placement, but maybe to the right of the "Cancel" button would work?

We can leave it be if it's not super easy to do, though.

[timabbott]

Please add the integration review label once you've fixed up these nits; I don't think we need to test-deploy this one.

[N-Shar-ma]

@zulipbot add "integration review"

[timabbott]

I'm not sure why 900 would be the better number to use than 999, but going to go ahead and merge the design change and I'll post a query about that detail in #design, since it's a tiny nit to change.