feat: terminal text selection and copy/paste functionality

* feat: terminal text selection and copy/paste functionality

- Add clipboard IPC handlers for readText/writeText operations
- Create useTerminalClipboard hook with selection state management
- Implement keyboard shortcuts: Ctrl/Cmd+C (copy), Ctrl/Cmd+V (paste), Ctrl/Cmd+A (select all)
- Add Radix UI context menu with Copy, Paste, Select All options
- Add rate limiting (100ms) for clipboard operations
- Add input validation and 10MB size limits for clipboard content
- Add accessibility labels and keyboard shortcut indicators
- Update terminal config with selection colors and screenReaderMode
- Add comprehensive test coverage (32 new tests)

Fixes:
- Register clipboard IPC in main process
- Fix context menu trigger for proper right-click behavior
- Fix stale closure issues in useTerminalClipboard hook
- Fix race conditions in async clipboard operations

Closes: terminal text selection and copy/paste feature

* fix: prevent double paste when using Ctrl+V keyboard shortcut

- Add isPastingRef guard to prevent concurrent paste operations
- Add event.preventDefault() to stop browser default paste behavior
- Add 100ms cooldown after paste to prevent rapid successive calls
- Fix duplicate import in use-terminal-clipboard.ts

Fixes issue where Ctrl+V would paste text multiple times while
context menu paste worked correctly.

* fix: use Buffer.byteLength for accurate clipboard size validation

Change text length validation from text.length (UTF-16 code units)
to Buffer.byteLength(text, 'utf8') to properly measure actual bytes.
This ensures multibyte characters (emoji, CJK) are correctly counted
towards the MAX_CLIPBOARD_SIZE limit.

Author: gnoviawan