Add notification history; pin ecosystem crates to v0.7.2

Keep a bounded in-memory history (last 50) and a layer-shell window
listing app, summary, truncated body, and time. Toggle with
`breadbar --history` or D-Bus `dev.breadway.Bar.ToggleHistory`.

Pin bread-theme, bread-utils, and bread-screenshots to tag v0.7.2.
bread-shared stays on bread v0.8.0-rc.1.

src/screenshot.rs

@@ -74,6 +74,11 @@ pub struct Cli {
     /// Capture canvas height — see `width`.
     #[arg(long, default_value_t = 1080)]
     pub height: u32,
+
+    /// Toggle the in-memory notification history on a running breadbar, then
+    /// exit. Keybind-friendly; does not start a second instance.
+    #[arg(long)]
+    pub history: bool,
 }
 
 pub struct ScreenshotRequest {