FileBrowser sidebar has no removable/network drives (quickAccessLocations is hardcoded)

## Description

The built-in file browser modal (`DankCommon/Modals/FileBrowser/FileBrowserContent.qml` in `dank-qml-common`) only ever shows a fixed set of quick-access locations in its sidebar:

```qml
property var quickAccessLocations: [
{ "name": "Home", "path": homeDir, "icon": "home" },
{ "name": "Documents", "path": docsDir, "icon": "description" },
{ "name": "Downloads", "path": downloadDir, "icon": "download" },
{ "name": "Pictures", "path": picsDir, "icon": "image" },
{ "name": "Music", "path": musicDir, "icon": "music_note" },
{ "name": "Videos", "path": videosDir, "icon": "movie" },
{ "name": "Desktop", "path": desktopDir, "icon": "computer" }
]
```

There's no enumeration of mounted removable media (USB drives, etc.) or network shares (SMB/NFS mounts via GVfs), so they never show up in the sidebar even when they're mounted and browsable via other file managers (e.g. Thunar with udisks2/gvfs).

## Request

Add dynamic entries to the sidebar for currently-mounted external/removable and network volumes, e.g. by querying `udisks2` (via D-Bus) and/or GVfs mounts, similar to what GTK's native file chooser and most file managers show under "Devices"/"Network". Even a simple always-refreshed list of current mounts (excluding the root/home filesystem) under a "Devices" section would cover the common case.

## Context

Noticed this while setting up DMS on niri: system-wide file pickers routed through `xdg-desktop-portal` (e.g. Firefox uploads) already show external/network drives fine via the GTK portal backend, since GTK's chooser does its own volume monitoring. It's specifically DMS's own internal file browser (used e.g. for wallpaper selection) that's missing this, since `quickAccessLocations` has no dynamic/mount-aware source.

Start with DankCommon/Modals/FileBrowser/FileBrowserContent.qml and trace how quickAccessLocations are rendered in the sidebar. Research the available udisks2 or GVfs mount-monitoring interfaces and how this project handles D-Bus or filesystem integration. Done means currently mounted removable and network volumes appear in a distinct sidebar section, excluding the root and home filesystems, and update when mounts change.

Written by the indexing model from the issue text.