Remove dead `useUserFileStore` Pinia store

## Recommendation

### 🗑️ SAFE TO REMOVE (after verification)

The `useUserFileStore` Pinia store is defined but never instantiated anywhere in the codebase. Only the `UserFile` class from the same file is imported by other modules.

## What Was Found

- Store is defined but `useUserFileStore()` is never called outside its own file
- The `UserFile` class from the same module IS actively used
- No dynamic imports or lazy references found

## How to Fix

1. Verify with: `grep -r 'useUserFileStore' src/ --include='*.ts' --include='*.vue'`
2. Remove the store definition from the file
3. Keep the `UserFile` class export

## If Removing (⚠️ proceed with caution)

- [ ] Verify no dynamic imports reference this store
- [ ] Check git blame — confirm this isn't recently added WIP

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**Category:** Dead Code | **Priority:** Recommended | **Effort:** Trivial
Part of #11022
_Filed by repo-audit skill_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11028-Remove-dead-useUserFileStore-Pinia-store-33e6d73d3650817fbbccec5aedffd7fe) by [Unito](https://www.unito.io)