Add Recoll full-text search plugin

[hyiltiz]

Summary

• Adds recoll as a submodule under plugins/recoll
• Provides filename search (r trigger) and full-text content search (rr trigger) via recollq CLI
• Queries the on-disk Xapian index maintained by Recoll, without loading any index into memory
• Configurable recollq path (auto-detected on macOS), result limit, debounce interval, and Recoll config directory via Albert settings UI
• MIME-type based icons with category fallback
• 4 actions per result: Open, Open folder, Copy path, Open terminal

Test plan

• Enable plugin in Albert Settings
• r <query> returns filename results with path as subtext
• rr <query> returns content results with abstract snippets
• Actions: Open, Open folder, Copy path, Open terminal
• recollq auto-detection on macOS
• Debounce prevents subprocess churn during rapid typing
• Graceful handling of missing binary, timeout, and no results

[hyiltiz]

The actual code for recoll plugin is a submodule. Need to identify the best practice for publishing those (so it won't get arbitrarily updated).