Media: Add a checkbox to toggle infinite scrolling in the media modal

[itzmekhokan]

Adds a point-of-use control for infinite scrolling in the attachments browser, as a follow-up to #65564.

What the problem was:

• Infinite scrolling is enabled by default as of [62632], but the only way to change it is the "Infinite Scrolling" option on the user profile screen. Users who need the "Load more" button for accessibility, or who want to avoid loading a large library, have to leave the modal, change their profile, and come back.

What the fix does:

• Renders a checkbox before the list of media items, in both the media modal and the Media Library grid view, that turns infinite scrolling on and off immediately.
• Unchecking it reveals the "Load more" button and the item count and stops the scroll handler from requesting more attachments. Checking it hides them and resumes loading on scroll.
• Keeps the screen reader search-results message in sync with the active mode.

Approach and why:

• The checkbox reflects the value resolved by wp_enqueue_media() and overrides it for the current view only, so the media_library_infinite_scrolling filter and the user profile option still determine the initial state and their documented precedence is unchanged.
• infiniteScrolling moves from a module-level constant to per-instance state on AttachmentsBrowser, and is passed down to the Attachments view rather than read from the global a second time.
• The scroll handler is now always bound and bails when infinite scrolling is off, so it is live if the user turns scrolling on mid-session.
• To make room for a control before the list, .attachments-wrapper is now the positioned, scrolling region in both modes rather than only when the "Load more" button is present. This reuses the offsets that already applied to the wrapper at every breakpoint, so no per-breakpoint pixel values change.

Trac ticket: https://core.trac.wordpress.org/ticket/65775

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