Unify Channel Settings UI and installed qualification

[dcellison]

Parent epic: channel settings and shared channel authority.

Problem

The final experience must feel like one compact settings surface rather than a stack of unrelated controls, while preserving the right panel's quieter navigation language and existing specialized People and Agents controls.

Scope

• Build the cohesive Channel Settings dialog using the canonical projection from the preceding issues.
• Organize it into compact General, Workspace, Participation, Access, and Lifecycle sections.
• Use the established dark theme tokens, typography, spacing, icon-only navigation/actions where appropriate, and the 1rem × 1rem right-panel icon rule.
• Show clear read-only states for members without edit authority.
• Keep People membership actions and Agents attachment actions in their existing right-panel sections.
• Place existing archive behavior under Lifecycle without adding permanent deletion.
• Replace No shared workspace with an actionable owner/admin empty state and an honest read-only member state.
• Preserve keyboard navigation, accessible names, focus management, responsive sizing, and remembered disclosure behavior where applicable.
• Reconcile the epic, add installation diagnostics, and write an installed qualification matrix covering authorization, multiple tabs, multiple principals/agents, workspace binding/revocation, guidance refresh, archival, and restart continuity.

Acceptance criteria

• Owners/admins can complete every authorized shared-channel setting workflow from the dialog.
• Members see the effective shared state and understandable reasons for read-only fields.
• The dialog does not contain personal notifications, unread/follow state, backend/model controls, personal workspace defaults, or duplicate People/Agents management.
• Empty, loading, stale-write, revoked-workspace, inaccessible-agent, and archived-channel states are explicitly covered.
• Focused client validation and the appropriate backend/integration suites pass.
• Installed qualification and make install-status demonstrate a clean canonical state.