Incomplete/missing ALT text when using Ctrl+Enter to post instead of clicking/tapping

**Describe the bug**
A clear and concise description of what the bug is.
- Which site: phanpy.social

**To Reproduce**
Steps to reproduce the behavior:
1. Click on the "new post" (feather icon) button
2. Paste an image
3. Click into alt-text area, fill out some text
4. Hit ctrl-enter

**Expected behavior**
Post is posted with alt text

**Screenshots**

The alt text is not "updated" into the model until "onblur" in the textarea, I'm guessing. This is probably not a problem when clicking/tapping the "Post" button, but it is when using Ctrl-Enter.

**Desktop:**
- OS: Windows
- Browser: Edge
- Version: 114.0.1823.43 (Official build) (64-bit)

**Additional context**

I've noticed that in the past with incomplete alt text, and just recently with the "missing alt text" warning and it clicked in my head

Reproduce the new-post flow with a pasted image, the alt-text textarea, and Ctrl+Enter, then trace the compose form's keyboard-submit handler and how the textarea value is committed. Done means posting with Ctrl+Enter preserves the entered alt text and matches the behavior of clicking or tapping Post.

Written by the indexing model from the issue text.