fix: detect available Linux terminal emulator instead of hardcoding gnome-terminal

[tusharbana-ansys]

Summary

Fixes #591 — Ansys Python Manager silently did nothing when clicking any action (Console, Install, Create venv, Uninstall, etc.) on Linux/WSL systems that don't have gnome-terminal installed, which is the common case on WSL.

Root cause

• execute_linux_command() in linux_functions.py hardcoded gnome-terminal for every Linux command execution, via a bare os.system(...) call whose exit status was never checked.
• WSL distributions (and many minimal Linux installs) don't ship gnome-terminal by default, so the command silently failed with no feedback to the user.
• Compounding this, the Ubuntu prerequisites (docs + installer.sh) told users to install the entire gnome desktop metapackage instead of just gnome-terminal.

Changes

• Terminal detection & fallback (linux_functions.py):
  • Added find_linux_terminal(), which detects the first available terminal emulator via shutil.which() from a prioritized list: gnome-terminal, konsole, xfce4-terminal, mate-terminal, tilix, xterm, x-terminal-emulator.
  • execute_linux_command() now uses whichever terminal is found, building the command with subprocess argv lists instead of os.system() (also fixes a latent shell-quoting bug from the old string-interpolated approach).
  • Raises a new NoLinuxTerminalError with a clear, actionable message (e.g. sudo apt-get install xterm) when no terminal emulator is available, instead of failing silently.
• Surface errors to the user instead of failing silently or misleadingly:
  • installed_table.py: Console/launch/package-management actions now show an error dialog via show_error() on failure.
  • uninstall.py: the uninstall confirmation flow previously had no error handling at all around the terminal launch — now wrapped and surfaced via show_error().
  • create_virtual_environment.py: venv creation previously showed a failure dialog and then unconditionally also showed a "successfully created" dialog right after — fixed to stop after the failure dialog.
• Fixed a related latent bug: find_miniforge_linux() passed a shell pipe (printenv | grep ...) to subprocess.check_output() without shell=True, so it always silently failed to detect system conda. Replaced with a direct os.environ["CONDA_PYTHON_EXE"] lookup.
• Docs & installer script: Ubuntu prerequisites now require gnome-terminal (not the full gnome desktop), matching what CentOS9/RHEL9/Fedora docs already said, plus a note recommending a lightweight xterm install for WSL.
• Tests: added regression tests for terminal detection and the no-terminal error path (tests/test_linux_functions.py).

Testing

• pytest tests/test_linux_functions.py tests/test_auto_updater.py tests/test_metadata.py — all passing.
• Verified end-to-end on real WSL distributions via WSLg (genuine Qt/xcb GUI, real button clicks), covering both the "no terminal" and "xterm installed" scenarios, across:
  • Ubuntu 22.04 (apt)
  • AlmaLinux 9 (RHEL9/CentOS9 family, dnf)
  • Fedora 43 (dnf)
• For each distribution: confirmed a clear error dialog (not a silent no-op) when no terminal emulator is present, and confirmed a real terminal process actually launches and the requested command completes once a terminal (e.g. xterm) is installed — for the Console action, virtual environment creation, and the uninstall flow.

Notes / follow-ups (out of scope for this PR)

• execute_linux_command() only verifies that the terminal launched, not that the wrapped script succeeded — pre-existing design gap (also applies to the Windows code path), not something we changed here.

[tusharbana-ansys]

Although automated tests have passed, I am still testing this manually on each supported distro. Please do not merge yet.
cc - @RobPasMue