Application layout shell (3-panel with resizable splitters)

[GordonBeeming]

Description

Main application layout with three resizable panels.

Layout

┌──────────────┬───────────────────────────┐
│              │      Query Editor         │
│   Object     │      (Monaco tabs)        │
│   Explorer   ├───────────────────────────┤
│   (tree)     │      Results Grid         │
│              │      (grid + messages)    │
└──────────────┴───────────────────────────┘

Acceptance Criteria

• Three-panel layout: left sidebar, center top (editor), center bottom (results)
• Resizable splitters between all panels (drag to resize)
• Splitter positions persisted to ~/.ssmsx/layout.json
• Collapse/expand sidebar with toggle button or Ctrl+B
• Minimum panel sizes enforced
• Responsive: panels adjust on window resize