Fix GUI crash when canceling file selection on startup

[Copilot]

Clicking cancel on the startup file dialog caused an IOError when attempting to read a non-existent config file.

Changes

• Detect cancellation: Check if filedialog.askopenfilename() returns empty string and load DEFAULT_CONFIG instead of attempting file read
• Display clear message: Set configfile = "No file selected" when user cancels instead of using a placeholder path
• Handle None values: Convert None to empty string in entry fields to avoid displaying "None" text
• Safe directory handling: Added get_config_dir() helper method that returns current directory when no file is selected, ensuring all file operations work correctly

# Before: Would crash if user canceled
configfile = prompt_file()
dic = aeolis.inout.read_configfile(configfile)

# After: Gracefully handles cancel
selected_file = prompt_file()
if selected_file:
    configfile = selected_file
    dic = aeolis.inout.read_configfile(configfile)
else:
    configfile = "No file selected"
    dic = DEFAULT_CONFIG.copy()

GUI now displays "No file selected" when user cancels, starts with empty fields, and allows users to configure manually or load a file later. All file dialogs use current directory as the initial location until a config file is loaded.

Original prompt

• when pressing cancel on startup no input file is specified. This gives an error now. Fix the error and load empty fields if no file is selected.

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[Sierd]

@copilot Remove the generic place holder, just state that no file is selected.

[Copilot]

@copilot Remove the generic place holder, just state that no file is selected.

Done. Removed the placeholder path and now configfile is set to "No file selected" when user cancels. Added a get_config_dir() helper method to safely handle directory operations when no file is selected. Changes in commit 68c62bb.