Design Desktop ECS canary opt-in, rollback, and bug-report flow

## Problem

The ECS soak needs an attributable Desktop cohort, but there is no explicit participation flow covering how users opt in, obtain the build, roll back, and report a bug with version context. Community messages alone do not provide that in-product lifecycle.

## Goal

Make Desktop canary participation safe, reversible, and measurable.

## Proposed Solution

Design the opt-in cohort flow, build acquisition/update behavior, one-step rollback, and structured bug reporting with automatic version attribution.

## Acceptance Criteria

- Participation is explicitly opt-in and cohort membership is measurable.
- Users can identify the active frontend/Desktop build.
- Rollback is documented and tested.
- Bug reports include version, mode, and relevant diagnostics by default.
- Ownership and go/no-go criteria are named.