Add a `/feedback` command

[AlhasanIQ]

Add a /feedback command that helps users report bugs or suggest features from within T3 Code.

It could open the relevant GitHub form with version and platform details prefilled for review before submission.

[BeeSting50]

Implemented this in #6570.

The PR adds /feedback on web, desktop, and mobile; opens the GitHub issue-form chooser with client metadata prepared for review; and uses Bug: / Feature: title defaults. Focused tests and targeted type checks pass.