theme, event_loop: window_controls.bar_bg slot + graceful exit on compositor disconnect

`WindowControlsSpec` gains `bar_bg`, the background fill of the SSD title-bar strip the close / maximize / minimize controls sit on. Forge used to paint that strip with `palette.surface` clamped to alpha 1.0 — a hack on a translucent panel token. With a dedicated slot the theme decides directly: `@off-white` in light and a new `@window-bar-dark` shade in dark. Schema gets the matching `Option<String>` field (defaulting to `palette.surface` to keep existing themes rendering the same) and the fallback `WindowControlsSpec` seeds `Color::WHITE` so a missing theme still draws something readable.
`try_run`'s dispatch loop previously `.expect("dispatch")`-ed every calloop error. When the compositor closes the wayland socket — `wl_display.error`, forge crashing, the user logging out — the loop saw a `BrokenPipe` / `ConnectionReset` and panicked, which polluted the user's stderr with a backtrace for what is just "the session ended". The match now treats those two `IoError` kinds as a clean shutdown: it prints `Connection::protocol_error()` (the typed `wl_display.error` if the server sent one), sets `exit_requested = true`, and lets the loop drain out normally. Any other dispatch error keeps panicking — those are still genuine bugs.

src/theme/schema.rs

@@ -740,6 +740,7 @@ struct RawLauncher
 #[ serde( deny_unknown_fields ) ]
 struct RawWindowControls
 {
+	#[ serde( default ) ] bar_bg:         Option<String>,
 	#[ serde( default ) ] icon:           Option<String>,
 	#[ serde( default ) ] hover_bg:       Option<String>,
 	#[ serde( default ) ] pressed_bg:     Option<String>,
@@ -814,6 +815,7 @@ fn window_controls_from_raw
 		Some( p ) => default_window_controls( *p ),
 		None      => WindowControlsSpec
 		{
+			bar_bg:         Color::WHITE,
 			icon:           Color::BLACK,
 			hover_bg:       Color::rgba( 0.0, 0.0, 0.0, 0.08 ),
 			pressed_bg:     Color::rgba( 0.0, 0.0, 0.0, 0.12 ),
@@ -824,6 +826,7 @@ fn window_controls_from_raw
 	};
 	Ok( WindowControlsSpec
 	{
+		bar_bg:         parse_opt( raw.bar_bg.as_deref(),         fallback.bar_bg         )?,
 		icon:           parse_opt( raw.icon.as_deref(),           fallback.icon           )?,
 		hover_bg:       parse_opt( raw.hover_bg.as_deref(),       fallback.hover_bg       )?,
 		pressed_bg:     parse_opt( raw.pressed_bg.as_deref(),     fallback.pressed_bg     )?,