Add folder thumbnail generation and AI re-tagging

[adaster98]

Description

When moving to RapidRAW, I encountered a situation where even in recursive view, i'd have to scroll up and down for hours to generate all thumbnails, and AI tagging would also run before viewport-driven thumbnail generation had produced every thumbnail which left most images without an AI tag.

This adds a folder level thumbnail generation button, as well as an optinal folder level AI-retag button (if AI tagging is enabled in settings).

Type of Change

• Bug fix
• New feature
• Breaking change
• Performance improvement
• Code refactoring
• Documentation update
• UI/UX improvement
• Build/CI or Dependency update

Changes Made

• add recursive folder-level thumbnail generation independent of the visible library grid
• report the complete folder image count through the existing thumbnail progress indicator
• add recursive AI re-tagging for folders while preserving user and color tags
• show the re-tag action only when AI tagging is enabled in settings

Test Configuration:

OS: Gentoo
Hardware: Ryzen 7, 32 GB RAM, AMD GPU

Tested by processing my entire lightroom library of over 20,000 photos. It worked flawlessly.

Checklist

• My code follows the project's code style
• I haven't added unnecessary AI-generated code comments
• My changes generate no new warnings or errors

AI Disclaimer:

Please state the involvement of AI in this PR:

• This PR is entirely AI-generated
• This PR is AI-generated but guided by a human
• This PR was handwritten with AI assistance (spell check, logic suggestions, error resolving)
• This PR contains only blood, sweat, and coffee (AI-free)