Fixes desktop message-fragment copying on macOS by adding an explicit copy-event fallback for selected chat text.

Changes

• Adds a focused message selection copy handler that writes the exact selected text to text/plain when a non-empty selection intersects rendered message content.
• Leaves native copy behavior untouched for text inputs, editable fields, collapsed selections, whitespace-only selections, and selections outside message content.
• Marks message bubbles as selectable/non-draggable so macOS/Wails drag regions do not interfere with text selection.
• Adds regression coverage for the copy decision boundaries and includes it in the frontend test scripts.

Verification

• git diff --check
• gofmt -l .
• npm run test:all
• npm run build
• go test ./...
• (cd desktop && go test ./...)
• Vite preview smoke test: loaded the desktop frontend, sent a mock message, and checked for console errors.

Closes #3861