fix: file picker appears behind connection form window

[datlechin]

Summary

• Removed window.level = .floating from the connection form window. .floating is designed for utility/inspector panels (Xcode inspector, Photoshop palettes), not form dialogs. This caused NSOpenPanel (which opens at .normal level) to appear behind the connection form.
• Converted all NSOpenPanel calls in connection form views from panel.begin { } to panel.beginSheetModal(for:) — the standard macOS pattern for file dialogs in form windows. The picker now slides down as a sheet attached to the form.

Closes #587

Test plan

• Open New Connection → select SQLite → click Browse → verify file picker appears as a sheet on the connection form
• Open New Connection → enable SSL → click Browse for CA/Client cert/key → verify file picker appears as a sheet
• Open New Connection → enable SSH → click Browse for private key → verify file picker appears as a sheet
• Open New Connection → SSH → add jump host → click Browse for jump host key → verify file picker appears as a sheet
• Open SSH Profile Editor → click Browse for private key → verify file picker appears as a sheet
• Verify the connection form window no longer floats above all other app windows