Session: fix page pointer handling in wait loop

- Refactor `wait` and `_wait` to handle `page` as `*Page` instead of `**Page`, preventing stale references during navigations.
- Update `networkidle` wait condition to use `_notified_network_idle == .done`.
- Document `--wait_ms` and `--wait_until` options in `Config.zig` help text.

src/Config.zig

@@ -398,6 +398,13 @@ pub fn printUsageAndExit(self: *const Config, success: bool) void {
         \\
         \\--with_frames   Includes the contents of iframes. Defaults to false.
         \\
+        \\--wait_ms       Wait time in milliseconds.
+        \\                Defaults to 5000.
+        \\
+        \\--wait_until    Wait until the specified event.
+        \\                Supported events: load, domcontentloaded, networkidle, fixed.
+        \\                Defaults to 'load'.
+        \\
     ++ common_options ++
         \\
         \\serve command