feat: command snippets + UI test coverage for new features
- core/snippets.py: SnippetManager with 20 prebuilt sysadmin commands,
CRUD + reorder, persisted to ~/.sshclient/snippets.json
- ui/snippet_dialog.py: SnippetDialog (add/edit) + SnippetManagerDialog
(table view with add/edit/delete/move up/down)
- ui/terminal_panel.py: snippet combo box in toolbar, send_command()
method to inject commands into interactive shell, snippet manager button
- README.md: updated feature table, usage section, project structure
- test_ui.py: added [2/4] checks for sparkline/snippets/theme,
added [4/4] theme toggle verification
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