Detect image URLs in post composer and display previews + add carousel picture counter badge

[alltheseas]

Summary

Detect image URLs in the post composer and display them as image previews. When users type or paste image URLs (jpg, jpeg, png, gif, webp, svg), the URLs are automatically detected and converted to image previews in the media carousel. The URL is removed from the post text to avoid duplication (it gets re-added when posting via the existing build_post() logic).

Features:

• Detect image URLs after whitespace/newline (when typing)
• Detect image URLs immediately on paste
• Show carousel position badge (e.g., "1/2") for multiple images
• Exclude fragment URLs (#) which don't return actual images (e.g., Wikipedia media viewer URLs)

Checklist

Standard PR Checklist

• I have read (or I am familiar with) the Contribution Guidelines
• I have tested the changes in this PR
• I have profiled the changes to ensure there are no performance regressions, or I do not need to profile the changes.
  • If not needed, provide reason: Changes are limited to post composer text processing with simple regex matching; no heavy computations or loops over large datasets
• I have opened or referred to an existing github issue related to this change: Feature Request: Recognize Pasted Image URLs as Images #2288
• My PR is either small, or I have split it into smaller logical commits that are easier to review
• I have added the signoff line to all my commits. See Signing off your work
• I have added appropriate changelog entries for the changes in this PR. See Adding changelog entries
• I have added appropriate Closes: or Fixes: tags in the commit messages wherever applicable, or made sure those are not needed. See Submitting patches

Test report

Device: iPhone 17 Pro Simulator

iOS: 26

Damus: 2bcef51

Setup: Fresh install on simulator

Steps:

1. Open post composer
2. Type an image URL (e.g., https://upload.wikimedia.org/wikipedia/commons/thumb/8/8a/Banana-Single.jpg/800px-Banana-Single.jpg) followed by a space
3. Verify URL is removed from text and image preview appears
4. Paste an image URL directly (without trailing space)
5. Verify image preview appears immediately
6. Add multiple images and verify "1/2", "2/2" badge updates when scrolling
7. Verify fragment URLs (e.g., https://example.com/page#image.png) are not detected

Results:

• PASS

Other notes

• Related to PR Hide image URLs in kind1 rendering #3372 which handles hiding image URLs after posting/in timeline display
• Unit tests added for regex pattern matching and URL extraction behavior

[alltheseas]