fix: convert all file open/save panels to sheet presentation

[datlechin]

Summary

Converts all remaining 10 instances of free-floating NSOpenPanel/NSSavePanel dialogs to sheet presentation via beginSheetModal(for:). This completes the window management audit items 1.1 and 1.2.

5 panel.runModal() → beginSheetModal(for:) (previously blocked the entire app):

• WelcomeViewModel: import connections file picker
• ConnectionExportOptionsSheet: export connections save panel
• ThemeListView: theme export and import panels (2 sites)
• InstalledPluginsView: plugin install file picker

5 panel.begin { } → beginSheetModal(for:) { } (previously free-floating):

• ImportDialog: data import file picker
• MainContentCoordinator+SidebarActions: SQL import file picker (uses contentWindow)
• LinkedFoldersSection: folder chooser
• ExportDialog: export save panel
• TableStructureView: DDL export save panel

Verification: grep -rn '\.begin {' TablePro/ returns zero panel matches. grep -rn 'runModal()' TablePro/ returns only alert fallbacks (correct pattern).

Test plan

• Welcome screen: File > Import Connections → file picker appears as sheet
• Welcome screen: Export connections → save panel appears as sheet
• Settings > Themes > Export → save panel as sheet on Settings window
• Settings > Themes > Import → file picker as sheet on Settings window
• Settings > Plugins > Install from file → file picker as sheet
• Import dialog > Browse → file picker as sheet
• Sidebar > Import SQL → file picker as sheet on main window
• Settings > Linked Folders > Add → folder chooser as sheet
• Export dialog > Export → save panel as sheet
• Structure > Export DDL → save panel as sheet