[Enhancement]: Implement a Fluent Design 2 (WinUI3) GUI.

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### Feature request

#### Which Nextcloud Version are you currently using:

I believe that I am unable to access this information.

#### Is your feature request related to a problem? Please describe.

WinUI versions 2 and 3 are the sole currently Microsoft-endorsed GUI toolkits for Windows applications. This matters, because like how on AOSP, any application targeting [API Level >31](https://developer.android.com/tools/releases/platforms#12:~:text=Platform%20Stability%20milestone.-,Android%2012%20(API%20levels%2031%2C%2032),-12L%20feature%20drop) is expected, by the userbase, to adhere to Material 3 design guidelines (the application style standard) and support Material You (the user-defined colouration customisation standard), Windows 11 users expect (Version > 1 of) WinUI.

Currently, NextCloud does not do so:


#### Describe the solution you'd like

At least, support the `FluentWinUI3` `QQuickStyle` on Windows 11.

#### Describe alternatives you've considered

Somehow implement the Fluent Design application style guidelines without doing so. This seems like a lot more work for no gain, though.

#### Additional context

OneDrive has done so:


Alternative examples of well-designed FD applications are:

1. [`simon-knuth/scanner`](https://github.com/simon-knuth/scanner/blob/e682515e384868e1171daf8254e908f3f2529a33/README.md#screenshots):


1. [`bloxstraplabs/bloxstrap`](https://github.com/bloxstraplabs/bloxstrap/blob/5d1aa50753002be94748e2394898042bc2bf0bcd/README.md):