feat: JSON viewer with Text/Tree toggle for query results

[datlechin]

Summary

Closes #752. Adds a full-featured JSON viewer for query results with two modes:

• Text mode: syntax-highlighted JSON with brace matching, auto-formatted on open, format button for re-indentation
• Tree mode: hierarchical tree view with expand/collapse disclosure triangles, search filtering, copy value/key path via right-click context menu

Both the grid popover (double-click a JSON cell) and the details pane (expand button on JSON fields) use the new viewer. The preferred view mode is persisted in Settings > Editor > JSON Viewer.

Changes

• 3 new files: JSONTreeNode.swift (data model + parser), JSONTreeView.swift (tree component), JSONViewerView.swift (container with toggle)
• Details pane: JSON fields show compact preview (120pt) with expand button that opens a full-height viewer with back navigation
• Grid popover: enlarged from 420x340 to 560x480, now uses the viewer with Text/Tree toggle
• Settings: new "JSON Viewer" section in Editor settings with default view mode picker
• Performance: tree parser capped at 5,000 nodes, 100K character input limit, O(1) string length checks

Test plan

• Open a table with JSON/JSONB columns
• Double-click a JSON cell — popover opens at larger size with Text/Tree toggle
• Switch to Tree mode — hierarchical tree with expand/collapse
• Use Expand All / Collapse All buttons
• Search within tree — filters nodes, auto-expands matches
• Right-click a tree node — Copy Value, Copy Key Path, Copy Key
• Open details pane — JSON field shows compact preview with expand button
• Click expand button — full-height JSON viewer replaces field list
• Click "Fields" back button — returns to field list
• Edit JSON in text mode, save — compacted JSON stored correctly
• Check Settings > Editor > JSON Viewer — default view mode persists across sessions
• Test with large JSON (>10K chars) — text mode works, tree may show truncation marker
• Test with invalid JSON — text mode works, tree shows error state
• Test with empty/null JSON values