Refactor floating menus to use HTML <dialog> and CSS popover with anchor()

[Keavon]

Modal dialogs (such as Preferences, About Graphite, closing save confirmations, etc.) should be refactored to use a <dialog> element.

Popover menus (such as menu lists and popovers) should be refactored to use the Popover API paired with CSS anchor positioning to replace the current CSS positioning hackery (which is fragile and isn't compatible with keeping up with scrolling its floating menu spawner within a pane with scrollbars). Care will need to be taken to ensure it works with nested popovers.

This is blocked on CSS anchor positioning reaching all modern browsers (caniuse). As of late 2024, Firefox and Safari do not support it.