Add external screenshot service support with configurable request delay

[Copilot]

The screenshot module previously required a local gowitness binary (and Chrome/Chromium). This adds support for external HTTP-based screenshot services with a per-request delay to avoid rate-limiting.

Backend

• New config keys: screenshot_service_url (default: https://image.thum.io/get/) and screenshot_delay (default: 2 seconds)
• New capture_screenshots_external(): fetches <service_url>/<target_url> per subdomain, sleeps screenshot_delay between requests, saves images locally with hash-suffixed filenames to avoid collisions
• capture_screenshots() routing: uses external service when screenshot_service_url is non-empty; falls back to gowitness otherwise
• update_config_settings(): validates and persists the two new keys

# Example: thum.io request for https://sub.example.com
GET https://image.thum.io/get/https://sub.example.com
# → saved as recon_data/screenshots/example.com/https_sub.example.com_a3f2c1d0.png

Settings UI

Two new fields added under the Screenshots section:

• External screenshot service URL — text input, placeholder https://image.thum.io/get/
• Delay between screenshot requests (seconds) — number input, min 0

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.