Match message copy control size to its metadata line

[witbrock]

Change

Size the Messages copy control to its surrounding metadata line. Inherit the line's text size, scale the icon to 1em, and use compact padding instead of the 32px desktop/44px touch minimum. Preserve the touch Copy label, tooltip and keyboard focus styling.

Validation

• Playwright Chromium fixture using current message metadata markup and the full candidate stylesheet, sent/received messages at 1280px desktop and 390px touch widths.
• Copy height: 32px → 13.52px desktop; 44px → 15.81px touch, beside 16px neighbouring controls. No horizontal page overflow.
• Inspected screenshot and git diff --check.
• Fixture evidence retained in the worker's .run/copy-control/; this is not authenticated server acceptance. CSS-only change; copy event handling is unchanged.

Native task: #V#task_agent_1065928a561a7d7668fbcc0dca5d5c46. Deployment is not requested.