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.

themes/default/theme.json

@@ -21,6 +21,7 @@
 		"navy":       "#0A032E",
 		"indigo":     "#221A5A",
 		"midnight":   "#181045",
+		"window-bar-dark": "#2B2838",
 		"white":      "#FFFFFF",
 		"off-white":  "#EAE9EB",
 		"sky":        "#2AA7F7",
@@ -93,6 +94,7 @@
 		"light": {
 			"launcher":        { "background": "@white/E6", "border_radius": 24.0 },
 			"window_controls": {
+				"bar_bg":         "@off-white",
 				"icon":           "@slate",
 				"hover_bg":       "@navy/14",
 				"pressed_bg":     "@navy/24",
@@ -206,6 +208,7 @@
 		"dark": {
 			"launcher":        { "background": "@indigo/E6", "border_radius": 24.0 },
 			"window_controls": {
+				"bar_bg":         "@window-bar-dark",
 				"icon":           "@silver",
 				"hover_bg":       "@white/18",
 				"pressed_bg":     "@white/26",