fix(shell): concentric hover-highlight radius on host cards

The FlowBoxChild draws the hover/selection highlight AROUND the card
(it wraps it with its own padding ring), so its default corner radius
ran visibly tighter than the card's 12px inside it. The host grids now
scope a 15px radius onto their children — card radius + the ring — so
the highlight's corners run concentric with the card's.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs-site/content/docs/hyprland.md

@@ -50,12 +50,23 @@ For how long the virtual output lives, and extend-vs-exclusive topology, see
 
 ## Requirements
 
-- A running Hyprland session (any recent release; validate on both a ≤ 0.54 and a ≥ 0.55 install).
+- A running Hyprland session (the `hyprctl`/xdph contracts are verified on **0.55.4**; older
+  releases share the same `hyprctl` surface).
 - **xdg-desktop-portal-hyprland (xdph)** installed and running — the host captures through its
   ScreenCast portal, and steers its custom picker. Without it there is no video.
 - The ScreenCast interface routed to xdph — see `scripts/headless/portals.conf` (a `[Hyprland]`
   section pins `org.freedesktop.impl.portal.ScreenCast=hyprland`).
 
+## Troubleshooting: black / no video (headless output at 0×0)
+
+A headless output only gets a framebuffer once the compositor can allocate one. On some GPU/driver
+combinations (notably NVIDIA, and in nested test setups) that GBM/dmabuf allocation fails and the
+output stays `0×0` — you'll see `GBM: Failed to allocate a GBM buffer: bo null` in the Hyprland log
+(cf. [Sunshine #4197](https://github.com/LizardByte/Sunshine/issues/4197)). The host detects this
+and fails the session with a clear error rather than streaming a blank surface. If you hit it,
+capture the Hyprland log (`hyprctl` instance dir → `hyprland.log`) and check your GPU's GBM support;
+running Hyprland as a real session (not nested) is the supported configuration.
+
 ## Permission system
 
 Hyprland's permission system (`ecosystem.enforce_permissions`, 0.49+, **off by default**) can deny