[Bug] Island media backdrop is offset and its corner radius doesn't match the island (backdrop overflows the island when the radius is increased)

### Compositor

Niri

### Distribution

Arch Linux

### If Other, please specify

_No response_

### Select your Installation Method

Distro Packaging

### Was this your original Installation method?

Yes

### If no, specify

_No response_

### dms doctor -vC

Click to expand
## DMS Doctor Report

**System**
- [ok] Operating System: Arch Linux
- ID: arch, Version: rolling, Arch: amd64
- [ok] Architecture: amd64
- [ok] Display Server: Wayland
- WAYLAND_DISPLAY=wayland-1

**Versions**
- [ok] DMS CLI: dms v1.6.1
- /usr/bin/dms
- [ok] Quickshell: Quickshell 0.3.1 (revision , distributed by Arch Linux)
- /usr/sbin/qs
- [ok] DMS Shell: v1.6.1
- /run/user/1000/danklinux-shell/d83248ba9119b48a

**Installation**
- [ok] DMS Configuration: Found
- /run/user/1000/danklinux-shell/d83248ba9119b48a
- [ok] shell.qml: Present
- /run/user/1000/danklinux-shell/d83248ba9119b48a/shell.qml
- [info] Install Type: Unknown
- /run/user/1000/danklinux-shell/d83248ba9119b48a

**Compositor**
- [ok] niri: 26.04
- /usr/sbin/niri
- [info] Active: niri
- [ok] Background Blur: Supported
- Compositor supports ext-background-effect-v1

**Quickshell Features**
- [ok] Polkit: Available
- Authentication prompts
- [ok] IdleMonitor: Available
- Idle detection
- [ok] IdleInhibitor: Available
- Prevent idle/sleep
- [ok] ShortcutInhibitor: Available
- Allow shortcut management (niri)
- [ok] BackgroundBlur: Available
- Background blur API support in Quickshell

**Optional Features**
- [ok] accountsservice: Available
- User accounts
- [ok] power-profiles-daemon: Available
- Power profile management
- [ok] logind: Available
- Session management
- [ok] cups-pk-helper: Available
- Printer management
- [info] I2C/DDC: No monitors detected
- External monitor brightness control
- [ok] qt6-imageformats: Installed (5 formats)
- Formats: WebP, TIFF, GIF, JP2, ICNS (/usr/lib/qt6/plugins/imageformats:/usr/lib/qt/plugins/imageformats:/usr/lib64/qt6/plugins/imageformats)
- [ok] kimageformats: Installed (4 formats)
- Formats: AVIF, HEIF, JXL, EXR (/usr/lib/qt6/plugins/imageformats:/usr/lib64/qt6/plugins/imageformats)
- [ok] Terminal: kitty, alacritty
- [ok] Network: NetworkManager
- NetworkManager present; iwd also running (likely NM's Wi-Fi backend). Using NM API.
- [ok] matugen: Installed
- Dynamic theming
- [ok] cava: Installed
- Audio visualizer
- [info] khal: Not installed
- Calendar events
- [info] danksearch: Not installed
- File search
- [info] dankcalendar: Not installed
- Calendar app
- [info] fprintd: Not installed
- Fingerprint auth
- [ok] adw-gtk3: Installed
- GTK3 dynamic theming (user copy)

**Config Files**
- [ok] settings.json: Present
- /home/ice_sheep/.config/DankMaterialShell/settings.json
- [info] clsettings.json: Not yet created
- /home/ice_sheep/.config/DankMaterialShell/clsettings.json
- [ok] plugin_settings.json: Present
- /home/ice_sheep/.config/DankMaterialShell/plugin_settings.json
- [ok] session.json: Present
- /home/ice_sheep/.local/state/DankMaterialShell/session.json
- [ok] dms-colors.json: Present
- /home/ice_sheep/.cache/DankMaterialShell/dms-colors.json

**Services**
- [ok] dms.service: enabled, active
- [info] greetd: Not installed
- Optional greeter service

**Environment**
- [info] QT_QPA_PLATFORMTHEME: qt5ct
- [ok] qt5ct plugin (Qt5): Installed
- /usr/lib/qt/plugins/platformthemes/libqt5ct.so
- [info] QT_QPA_PLATFORMTHEME_QT6: qt6ct
- [ok] qt6ct plugin (Qt6): Installed
- /usr/lib/qt6/plugins/platformthemes/libqt6ct.so
- [info] QS_ICON_THEME: Adwaita
- [info] XDG_MENU_PREFIX: Not set

**Fonts**
- [ok] Normal Font: HarmonyOS Sans SC
- Available via fontconfig
- [ok] Monospace Font: JetBrains Maple Mono
- Available via fontconfig

---
**Summary:** 0 error(s), 0 warning(s), 35 ok

### Description

When the media player expands inside Dank Island (with the album-art wallpaper/backdrop enabled):

1. **The backdrop is offset**: the blurred album-art backdrop is shifted relative to the island itself and is not aligned/centered with the media content.
2. **The corner radius doesn't match the island**: the backdrop's corner radius is smaller than the island's, so the edges don't line up.
3. **The backdrop overflows when the radius is increased**: after increasing the island Corner Radius, the backdrop's corners stick out past the island's rounded outline.

See attached screenshot (Image 1): the media backdrop clearly mismatches the island corners and is offset as a whole.

### Expected Behavior

- The backdrop should line up exactly with the island body — no offset, centered.
- The backdrop's corner radius should always equal the current island corner radius, updating when the radius is changed, and should never extend past the island outline.

### Steps to Reproduce

- Enable Dank Island and enable the "Wallpaper" toggle in Settings → Media Player.
- Play any track (any MPRIS player).
- Open / expand the island media player.
- Observe the position and corner radius of the blurred backdrop versus the island body.
- Increase the island Corner Radius (Settings → Island → Appearance) and observe again.

### Error Messages/Logs

_No response_

### Screenshots/Recordings

Start at the Dank Island media player entry point and the Settings → Media Player wallpaper toggle, then trace how the expanded island backdrop is positioned and rounded. Reproduce with an MPRIS player, compare the backdrop with the island body while changing Island → Appearance → Corner Radius, and consider the issue done when alignment, radius, and clipping remain correct.

Written by the indexing model from the issue text.