niri: float and center Steam client and child windows

Add window rules for the Steam client (app-id "steam"): all client
windows float and center, the main library window opens large (75x85%),
and notification toasts sit unobtrusively in the bottom-right without
border or focus-ring. Launched games (steam_app_) are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

niri/config.kdl

@@ -157,3 +157,30 @@ window-rule {
     match app-id="^thunar$" title="^File Operation Progress$"
     open-floating true
 }
+
+// Steam client + child windows: float and center (friends list, settings,
+// game properties, dialogs). Games (steam_app_) are handled above, unaffected.
+window-rule {
+    match app-id="^steam$"
+    open-floating true
+}
+
+// Steam main library window: float large (centered by niri)
+window-rule {
+    match app-id="^steam$" title="^Steam$"
+    default-column-width { proportion 0.75; }
+    default-window-height { proportion 0.85; }
+}
+
+// Steam notification toasts: unobtrusive, bottom-right, no border/focus-ring
+window-rule {
+    match app-id="^steam$" title="^notificationtoasts"
+    open-floating true
+    default-floating-position x=16 y=16 relative-to="bottom-right"
+    border {
+        off
+    }
+    focus-ring {
+        off
+    }
+}