host: hidden tab webviews use the WebView2 low memory target

set_tab_visible shows/hides a tab webview and sets
ICoreWebView2_19::SetMemoryUsageTargetLevel (LOW when hidden, NORMAL when
shown). Hidden tabs keep running; the browser trims their working set.

crates/waybetter-host/src/tabs.rs

@@ -159,6 +159,23 @@ impl TabAction {
     }
 }
 
+/// WebView2 memory target for a tab webview. A hidden tab (background tab of a group, or a pane
+/// clamped away) asks the browser to drop caches, decoded images and GPU tiles it can rebuild;
+/// its page keeps running (timers, sockets, notifications). Shown tabs go back to normal.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum MemoryTarget {
+    Normal,
+    Low,
+}
+
+pub fn memory_target(visible: bool) -> MemoryTarget {
+    if visible {
+        MemoryTarget::Normal
+    } else {
+        MemoryTarget::Low
+    }
+}
+
 /// Hands a JSON value to a member of the page runtime: `window.<global>.<member>.<method>(value)`
 /// when the runtime exists, else parked in `window.<queue>` for the runtime to pick up at start.
 /// `json` must be serialized JSON (never page-supplied text).
@@ -221,6 +238,12 @@ mod tests {
         assert_eq!(TabState::reported("tab-1".into(), None, None, Some(long_url), None).favicon, None);
     }
 
+    #[test]
+    fn hidden_tabs_target_low_memory() {
+        assert_eq!(memory_target(false), MemoryTarget::Low);
+        assert_eq!(memory_target(true), MemoryTarget::Normal);
+    }
+
     #[test]
     fn layouts_default_to_the_default_profile() {
         let layout: TabLayout = serde_json::from_str(r#"{"id":"a","url":"https://a.test/","x":0,"y":0,"width":1,"height":1,"visible":true}"#).unwrap();