fix: allow pasting file paths in file loading dialog

[patrickoleary]

Enable pasting file paths directly into the simulation and connectivity text fields in the file loading dialog.

• Removed disabled=True from both the Simulation File and Connectivity File text fields
• Added placeholder text: "Paste or browse for simulation/connectivity file path"

Users can now paste a full file path instead of navigating the file browser tree.

Note: Probably needs more error handling

• Selecting via the dialog seamlessly handles path errors, but connectivity/simulation mismatches are not handled.
• Pasting path-filenames there is no seamless error handling for paths or connectivity/simulation mismatches.

closes #77