Consolidate settings and add safe chat history deletion

[JRH89]

Changes

• One Settings page for workspace policy and repository settings; new manual runs inherit the configured budget.
• Persist setup banner dismissal per identity/workspace/browser, with right-aligned actions.
• Preserve host ports in local OAuth proxy callbacks.
• Delete chat history from Recent chats cards after confirmation; enforce ownership/workspace checks, reject running replies, retain accounting and GitHub work.
• Document behavior and regression coverage.

Verification

89 frontend tests, lint, typecheck, build and SEO checks passed. Five populated layout tests passed. Backend service deletion tests and full Docker Maven verification passed. Local web/API rebuilt and healthy.

Closes #120