[Bug]: File watcher thread slows down whole application

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I've searched it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

The file watcher thread gets over-pressured by spurious notifications on Linux.

Running on Arch Linux, present in the most recent Nextcloud desktop release (tried even git version), with an account that has 220k+ files (Server ZFS and client ext4).

```
Nextcloud version 3.16.50daily
Git revision 120505be92d1aa9ffe31a4fd5e4d187b10bf4602
Using Qt 6.8.2, built against Qt 6.8.2
Using Qt platform plugin 'wayland'
Using 'OpenSSL 3.4.1 11 Feb 2025'
Running on Arch Linux, x86_64
```

I do not know if it could also be a inotify problem (from the code it seems it might be, there are no mentions of Windows but mentions of MacOS not having that issue), and since it even is acknowledged by the person who wrote the code, there is counter-measures to discard a file that has not changed. Which is great! But since it does check, it also writes thousands upon thousands of log lines, running the operation to a slog. To see if it would help, I changed the qcInfo call to a qcDebug, recompiled in Release mode and the application went from being unresponsive for hours to... being unresponsive for a few minutes. Which is a **major** improvement!

But this does raise an important question: why isn't that file watching thread being run in a separate thread as the main one? If it is so computationally intensive task, running it in the background and maybe writing to the log files the number of spurious notification met would keep the main application responsive and the experience much more smooth. I do understand the value of seeing per-file spurious notification discard, but this tie into major issues where users have hundreds of MB of logs written on every sync. Being able to set the debug level to write to logs in the application could also be a solution to this situation.

The offending code : [slotWatchedPathChanged](https://github.com/nextcloud/desktop/blob/120505be92d1aa9ffe31a4fd5e4d187b10bf4602/src/gui/folder.cpp#L595) and more specifically [this line that prints heavily on the log file](https://github.com/nextcloud/desktop/blob/120505be92d1aa9ffe31a4fd5e4d187b10bf4602/src/gui/folder.cpp#L673)

I hope to see that line changed from qcInfo to qcDebug in the next release so I can finally use VFS on my laptop. In the long-term, I hope that this process can be multi-threaded to avoid the main applet being unresponsive during this operation.

Anyway, thanks for making such a great self-hosting cloud application!

### Steps to reproduce

1. Start Sync
2. Tail the created logs
3. See thousands of lines of "Ignoring spurious notification for file XYZ"
4. The nextcloud client is unresponsive for how long it takes it to both discard the notifications and write the logs

### Expected behavior

1. Start sync
2. See "Verifying ## **possible** changed files..."
3. The nextcloud client should stay responsive
4. See in the info logs "Ignored ## spurious notifications" when the sync is finished

OR

1. Being able to raise log level to warning and above from the application

### Which files are affected by this bug

src/gui/folder.cpp

### Operating system

Linux

### Which version of the operating system you are running.

Linux 6.13.2-arch1-1

### Package

Compiled it myself

### Nextcloud Server version

30.0.4

### Nextcloud Desktop Client version

3.16-rc1

### Is this bug present after an update or on a fresh install?

Fresh desktop client install

### Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

### Are you using an external user-backend?

- [x] Default internal user-backend
- [ ] LDAP/ Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

_No response_

Start in src/gui/folder.cpp, especially slotWatchedPathChanged around lines 595 and 673, and reproduce the issue by starting a sync while tailing the logs. Compare the per-file spurious-notification logging with the reported expected behavior: the client remains responsive and the logs report an aggregate count after syncing, or the application provides a way to raise the log level.

Written by the indexing model from the issue text.