refactor(calendar): replace subscription link with button

[lucs7]

Based on @labmecanicatec suggestion follow up to #1644

This pull request refactors the calendar subscription UI to use a <button> element instead of an <a> tag for the "Subscribe to Calendar" action. This change improves accessibility and ensures proper disabling of the button while new calendar data is loading. Related CSS and JavaScript logic have been updated to match this new approach

[Copilot]

Pull request overview

Refactors calendar subscription controls from links to native buttons for proper disabled-state handling during refreshes.

Changes:

• Replaced subscription anchors with buttons in both templates.
• Updated JavaScript to use the native disabled property.
• Removed obsolete anchor-specific CSS.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Web/scripts/calendar.js	Disables the subscription button during refreshes.
Web/css/librebooking.css	Removes obsolete anchor-disabled styling.
tpl/Calendar/mycalendar.subscription.tpl	Uses a button for personal calendar subscriptions.
tpl/Calendar/calendar.subscription.tpl	Uses a button for calendar subscriptions.

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[labmecanicatec]

I think it's appropriate to change the title attribute in tpl/Admin/Resources/manage_resources_public.tpl lines 17 and 22 to {translate key=SubscribeToCalendar}