fix(terminal): copy selected text with Ctrl+C

[jackie-cqz]

Summary

• intercept plain Ctrl+C only when the xterm instance has an active selection
• copy the exact selected text through the shared clipboard helper and prevent ETX/SIGINT in that case
• preserve normal terminal handling when there is no selection or when Shift/Alt/Meta is present
• add focused regression coverage for lowercase/uppercase C, selection state, event phase, and modifier gates

Verification

• pnpm exec vitest run tests/terminal-keybindings.spec.ts (9/9 passed)
• pnpm typecheck
• pnpm exec eslint src/client/TerminalView.tsx src/client/terminal-keybindings.ts tests/terminal-keybindings.spec.ts
• pnpm build
• full pnpm test: 1243 passed; the remaining local-only failures were Windows environment limitations (node-pty AttachConsole passed when rerun in a TTY; one existing symlink test cannot create a symlink without Windows Developer Mode/privilege)

Closes #465