control panel: real padding, seat grab, key hints

- GtkBox set_border_width shrinks the widget's own background, so the card was
  inset 30px while its contents sat flush against the card edge. Padding is css
  now: 12px transparent gutter outside, 20px inside. Measured on a virtual
  display rather than guessed.
- the panel grabs the seat on map, like rofi, so it keeps keyboard and pointer
  focus wherever the cursor is; the grab is released on close
- every control shows its key in the bottom-right: n b v d p w for the tiles,
  a c s t for the row beneath, l o z r q for power, m/i to mute, arrows for
  volume

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cn3dHfbgNYdbZvzb1b3GQt

.config/rice/README.md

@@ -51,7 +51,20 @@ sliders are live and stepping back between views never redraws the panel:
 | Wallpaper / Particles | shuffle, toggle the particle layer |
 | Power | lock, log out, suspend, reboot, shut down |
 
-Escape steps back from a subview; Escape on the main view closes it.
+Escape steps back from a subview; Escape on the main view closes it. The panel
+grabs the seat like rofi does, so it keeps focus wherever the pointer is.
+
+### Panel keys
+
+Each control shows its key in the bottom-right corner.
+
+| | | | |
+|---|---|---|---|
+| `n` network | `b` bluetooth | `v` vpn | `d` do-not-disturb |
+| `p` particles | `w` wallpaper | `a` audio | `c` clipboard |
+| `s` screenshot | `t` toggle bar | `m` mute | `i` mute mic |
+| `l` lock | `o` log out | `z` suspend | `r` reboot |
+| `q` shut down | `←/→` volume | `Esc` back / close | |
 
 ## Keybindings