feat: Add hierarchical tag grouping with collapsible headers

[reddraconi]

Adds a feature to group thumbnails in the main window by a tag or by a parent tag.
Grouped thumbnails in each section respect the user's other sorting choices (filename/date created, sort direction, etc.)

Features:

• Hierarchical tag dropdown with grouping of parent/children tags in a table view so large amounts of tags can be handled (hopefully)
• Collapsible/expandable groups with count badges in thumbnail view
  • The tags that act as headers respect the user's configuration and use the tags' configured colors
• Auto-refreshes the thumbnail pane and group-by-tag dropdown on tag creation/modification no matter if it was through the tag manager or the right-hand pane
• Works with all existing search/filter operations

Core Library Changes:

• Add group_entries_by_tag() method to generate a tag hierarchy
• Add get_grouping_tag_ids() to fetch grouping tags
• Extend BrowsingState with group_by_tag_id field
• Add GroupedSearchResult dataclass with TagGroup hierarchy

UI Components:

• Added a new dropdown for tag selection to sort by. This works with parent tags so multiple tag groups are created in the thumbnail view. If an item has multiple sibling tags, it'll be put under a "Multiple Tags" grouping.
  • Each Parent gets its own row in the dropdown menu. The siblings under the parent get their own cell. I didn't have a good way to screenshot that, sorry!
• Implemented GroupHeaderWidget to create collapsible group headers
• Extended ThumbGridLayout to render hierarchical grouped entries
• Updated update_browsing_state() to handle grouped results

Files Added:

• src/tagstudio/qt/mixed/group_header.py
• src/tagstudio/qt/mixed/group_by_tag_delegate.py

Notes:

• The QT components and the hierarchy generation were written with tool-assistance (Claude Code 4.5)
• The new python code was run through black -l100, mypy, and ruff. I did my best to stick to the existing coding style.
• I've tested this for hours yesterday and today with no ill effects (yet).
• I asked in the Discord if this had a PR and received positive responses there was one somewhere. I wasn't smart enough to find it, so if this is out of line, please let me know!
• I added 'Thumbs.db' to the ignore list since other Windows and Mac library-ish things were being ignored too.
• I found a bug where the search modal had delete buttons for tags, they didn't do anything because the delete_tag() only contained pass. I implemented a full delete_tag() method to fix this so the sort-by-tag dropdown would be able to automatically update correctly.

Summary

Adds a feature to group thumbnails in the main window by a single tag or by a parent tag.
Grouped thumbnails in each section respect the user's other sorting choices (filename/date created, sort direction, etc.)

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable
  • Ruff, mypy, and formatting compliance

Screenshots

New Toolbar Dropdown

Selecting a Parent Tag to Group By

Sorting complete

Example Item with Multiple Sibling Tags

[Demohstens]

This is a QT thing so I would suggest adding the method to the ignore list for this rule.

[reddraconi]

I added a quick check for Mock in the new code to prevent the math issues that occurred. I also went back and fixed up the linting issues caused by QT convention.