fix(ui): keep button text visible during loading states and improve accessibility

[TeeYml]

PR Description

Problem
The shared Button component was hiding its children while isLoading was true, only showing a spinner. This caused important context to be lost in several flows (e.g., whether a project was being submitted, updated, or loaded).

Solution
This PR updates the Button component to retain its meaningful text during loading states by default, while still ensuring forms prevent duplicate submits.

Key Changes:

• Preserved Context: The button's label (children) is no longer hidden when isLoading is true. The spinner now gracefully replaces only the leftIcon.
• Improved Accessibility:
  • Added aria-busy={isLoading} to communicate the loading state to screen readers.
  • Added aria-hidden="true" to the Loader2 spinner to prevent repetitive or confusing screen reader announcements.
  • Introduced an optional loadingText prop. If a button lacks explicit text or an aria-label, a screen-reader-only (sr-only) fallback text is automatically rendered to ensure strict accessibility compliance.
• Backwards Compatible: Existing callers will continue to work without layout shifts. Duplicate submissions remain prevented as the disabled={isLoading || disabled} behavior was explicitly retained.

Closes #65