Feature: Sidebar tag grouping using tree layout

### What feature or improvement do you think would benefit Files?

I work across many different creative fields: concept art, 3D modeling, video production, photography, and more. Managing project files across all of these has been genuinely painful. I've tried Adobe Bridge (clunky and slow), XnView, Eagle, Connecter, and other DAM tools, but nothing really hits the mark. They're either too heavy, too confining, or just not designed for how I actually work.

What I really wish for is a nested tag system built right into a good file manager. Files already does so much right, but the current tagging is flat, and once you have more than a handful of tags it becomes hard to organize and use them effectively.

**What I'm proposing:**

Let tags have parent-child relationships, so you can build a tree like:

- Work > Concept Art > Characters, Environments, Props
- Work > 3D Models > Characters, Vehicles, Environments
- Work > Videos > B-Roll, Final Cuts, WIP
- Personal > Movies > Watch Later, Favorites
- Personal > Games

Then give this tag tree a proper UI. A dedicated page or sidebar section where you can browse and navigate through your tag categories, expand into sub-tags, and select what you want to see.

**The features that would make this actually useful:**

1. Tags can have sub-tags, forming a tree structure. Existing flat tags just become root-level tags, no migration needed.
2. A tag tree navigator. Click a parent tag to see all files under it and its children. Think of it like folder browsing but for tags.
3. Multi-tag selection with AND/OR filtering. Select "3D Models" + "Environment" and only see files that match both. Or switch to OR mode to see everything from either.
4. The home page widget could show root tag categories as cards, each showing a file count. Click to dive in.
5. Pin your most-used tags to the sidebar for quick access.
6. A cleanup/reject tag, similar to how Lightroom lets you flag photos for deletion. Mark files throughout your session, then batch-delete them all at once instead of hunting them down one by one.

**Why this matters:**

This would literally save me so much pain. Right now I spend way too much time looking for files, manually sorting things into folders, and losing track of assets that belong to multiple projects. A proper hierarchical tag system with a dedicated navigation page would turn Files into my main file powerhorse and I would be able to abolish all other tools and stop suffering. Thx.

### Requirements

- Tags can have a parent tag (null = root), forming a navigable tree hierarchy
- Tag tree navigator view in sidebar or dedicated page with expand/collapse
- Multi-tag selection with AND/OR toggle for intersection/union queries
- Home page widget shows root tag categories as cards with file counts
- Pinnable tags for quick sidebar access
- Cleanup/reject tag with batch delete capability (Lightroom-style flagging)
- Backward compatible: existing flat tags become root tags, no migration needed

### Files Version

4.0.38.0