feat: support drag/paste image uploads and add a localize-images helper

[pfeerick]

Summary

• Localize helper (tools/download_external_images.py, new): the issue templates already tell contributors "Local ASSETS/ screenshots can be added by a maintainer after review," but there was no tool for that step — it was fully manual. This downloads any external image URLs still sitting in scripts.json into ASSETS/<slug>/screenshot-N.<ext> (matching the existing naming convention) and rewrites scripts.json to point at the local copies. --dry-run previews without writing; a failed image leaves its entry untouched rather than partially migrating it; an entry's existing slug (from any already-local images) is reused so a renamed entry doesn't split across two directories.
• Drag/paste upload support: GitHub issue-form textareas natively support dragging or pasting an image directly into a field — GitHub uploads it and inserts Markdown image syntax (![image](https://github.com/user-attachments/assets/<uuid>)) at the cursor. The "Image URLs" field's parser only ever recognized bare URL lines, so this was previously silently dropped. extract_image_urls() now finds URLs anywhere in the text, not just at the start of a line, and both templates' descriptions mention the drag/paste option explicitly.
• Because GitHub's upload-attachment URLs have no file extension in the path at all (unlike a raw GitHub URL ending in .png), the localize helper's extension detection falls back to sniffing the response's Content-Type header when the URL alone isn't enough.

Test plan

• extract_image_urls() against bare URLs, Markdown-wrapped upload URLs, and mixed text — all extracted correctly
• issue_to_scripts.py --mode insert with a Markdown-wrapped image URL — accepted; with genuinely invalid image text — still correctly rejected
• download_external_images.py end-to-end against a scratch copy of scripts.json, real network download of the one entry (EdgeTX Flight Log) that currently has external URLs — all 4 images downloaded, verified as real PNGs, scripts.json correctly rewritten to local paths
• uv run tools/test_sync_issue_template_options.py — 10 tests pass (unrelated to this change, confirming nothing else regressed)
• uv run tools/validate_scripts.py / sync_issue_template_options.py --check — clean
• YAML validity of both templates

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