feat: add JSON results view mode

[datlechin]

Summary

Closes #856

Adds a first-class JSON view mode to the results area, following Apple's view mode pattern (like Finder's List/Icon/Column or Xcode's plist Source/Property List toggle).

• Replaces showStructure: Bool with ResultsViewMode enum (.data, .structure, .json)
• Table tabs: Data / Structure / JSON segmented picker in status bar
• Query tabs: Data / JSON picker (no Structure for queries)
• Reuses existing JSONViewerView (text + tree modes) and JsonRowConverter (type-aware serialization)
• Shows selected rows only when a selection exists, all rows otherwise
• Read-only with Copy JSON button
• Per-tab state (switching tabs preserves view mode)

Test plan

• Table tab: status bar shows [Data] [Structure] [JSON], click JSON to see formatted JSON
• Query tab: run a query, status bar shows [Data] [JSON]
• Select 2-3 rows in Data mode, switch to JSON, verify only selected rows shown
• Deselect all, switch to JSON, verify all rows shown
• Toggle Text/Tree modes in JSON viewer toolbar
• Click Copy JSON, paste in editor, verify valid JSON with correct types
• Empty query tab (no results): JSON shows "No Data" placeholder
• Switch between tabs, verify JSON mode is per-tab
• Structure view still works as before