Add copy message action

[grapeot]

Summary

• add Copy message to both user and assistant message overflow menus
• copy all non-empty text parts as complete raw Markdown, separated by blank lines
• use the current suspend clipboard API and disable the action when no message text exists
• add English and Chinese localization plus a focused multi-part copy test

Verification

• ./gradlew assembleDebug testDebugUnitTest passed
• git diff --check passed