fix: add image link to the discovery queue

[SuoweiHu]

Problem

Unless manually provided an image link to crawl in the first place, images almost never shown in the Images tab, despite the crawler extracting image metadata from HTML pages. For mainly two reasons:

1. extract_links() only queued <a href> URLs — <img src> URLs were collected for the Links tab (collect_all_links()) and for per-page metadata (extract_images()), but were never added to the crawl discovery queue. So image URLs were never actually fetched as standalone resources.
2. Default include_extensions excluded image types (jpg, jpeg, png, gif, webp, svg, ico, avif, bmp) — Even if image URLs had reached the queue, the _should_crawl_url() extension filter only allowed html, htm, php, asp, aspx, jsp, rejecting all image file extensions.

Changes

File
crawler.py	Added jpg, jpeg, png, gif, webp, svg, ico, avif, bmp to the default include_extensions list in _get_default_config()
settings_manager.py	Same image extensions added to the default includeExtensions string
settings.js	Same image extensions added to the JS-side defaultSettings.includeExtensions
index.html	Updated the <input> default value for the Include File Extensions field to include image types
link_manager.py	Added a second pass in extract_links that iterates over <img src> tags after the existing <a href> loop. Each image URL is queued via the same should_crawl_callback filter to ensure they fulfil the includeExtension constraint.

Notes

Before/After Change Comparison (crawling the same URL):