[Feature]: Web UI: Allow closing task modal by clicking backdrop (outside modal content)

[eyanq]

Is your feature request related to a problem? Please describe.
Yes. In the web UI task details modal, mouse users currently must click the top-right × button to close the modal.
Although Esc already closes the modal in preview mode, clicking outside the modal content (on the backdrop) does nothing. This creates an inconsistent interaction pattern between keyboard and mouse, and makes quick task review slower and less intuitive.

Describe the solution you'd like
Add backdrop-click-to-close behavior for the task details modal.
Specifically, I would like backdrop click to follow the same close policy as Esc:

• If Esc is allowed to close the modal, clicking the backdrop should also close it.
• If Esc is intentionally disabled (for example in create/edit states with unsaved changes), backdrop click should be disabled as well (or follow the same confirmation flow).
  This keeps behaviour consistent, predictable, and aligned with common modal UX patterns.

Additional context
Typical expected modal behavior in modern web apps is:

• click outside modal => close (when safe)
• Esc => close (when safe)
• restricted editing mode => both are restricted consistently

Current behaviour in Backlog web UI is that only Esc and × work in preview mode, while backdrop click does not.
This is especially noticeable during rapid triage, where users repeatedly open/close tasks and naturally try clicking outside first.