Add optional copy buttons for fenced code blocks

[geauxtigers]

Summary

This PR adds an optional copy content setting that shows copy buttons on fenced code blocks.

Changes

• adds content.copy default and migration handling
• adds a popup label for the new setting
• renders copy buttons for syntax-highlighted fenced code blocks
• copies the live code block text to the clipboard
• trims trailing CR/LF from copied content so the clipboard ends at the last printable character
• toggles a visual marked state on successful copy
• documents the new option in the README

Behavior

• the feature is off by default
• buttons are shown only when copy is enabled
• successful copy toggles the button between its base state and a green marked state
• the copied text preserves indentation and internal blank lines, but drops trailing line endings

Scope

This PR intentionally does not include the unrelated background-script guard fixes, which are proposed separately.