Fix memory leak in resize divider initialization

[Copilot]

The setup_resize_divider() function in ViewPage was called on every effect trigger without guards, causing event listener re-registration and memory leaks.

Changes:

• Added is_mounted signal guard to ensure one-time initialization
• Set flag before requestAnimationFrame to prevent race condition where multiple callbacks queue before first executes
• Follows existing pattern from create.rs

Before:

create_effect(move |_| {
    if let Some(window) = web_sys::window() {
        let callback = Closure::once(move || {
            setup_resize_divider();
        });
        window.request_animation_frame(callback.as_ref().unchecked_ref()).ok();
        callback.forget();  // Called on every effect trigger
    }
});

After:

let (is_mounted, set_mounted) = create_signal(false);

create_effect(move |_| {
    if !is_mounted.get() {
        set_mounted.set(true);  // Guard prevents redundant calls
        if let Some(window) = web_sys::window() {
            let callback = Closure::once(move || {
                setup_resize_divider();
            });
            window.request_animation_frame(callback.as_ref().unchecked_ref()).ok();
            callback.forget();
        }
    }
});

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