fix(hig): use NSSplitView for Server Dashboard panels (#1464)

[datlechin]

Summary

Fixes #1464 — the Slow Queries panel at the bottom of the Server Dashboard could not be opened.

Root cause: the dashboard laid out Active Sessions, Server Metrics, and Slow Queries inside a plain VStack. SwiftUI's Table (in SessionsTableView) is greedy and consumed all vertical space, so the DisclosureGroup for Slow Queries got 0 pt of content area. Apple's own developer forum #726444 documents this exact DisclosureGroup-with-greedy-content failure.

Fix: replace the inner VStack with NSSplitView (vertical orientation, thin dividers) per Apple HIG guidance for multi-pane dashboards. The split view lives in a new ServerDashboardSplitView that wraps NSSplitViewController via NSViewControllerRepresentable, mirroring the existing QuerySplitView.swift pattern. splitView.autosaveName persists divider positions across launches.

The inner DisclosureGroup in SlowQueryListView is removed — the split divider is the resize affordance and the three panels now share a uniform header style.

What's in the diff

• ServerDashboardSplitView.swift (new) — NSSplitViewController wrapper. Items added in panel order from viewModel.supportedPanels, so SQLite/DuckDB (one pane) and MySQL/PostgreSQL/MSSQL/ClickHouse (three panes) both work.
  • Sessions: min 120 pt, low holding priority (grows with the window).
  • Metrics: min 76 / max 200 pt, higher holding priority.
  • Slow Queries: min 100 pt, canCollapse = true.
• ServerDashboardView.swift — drops the inner VStack and manual dividers; uses the new wrapper.
• SlowQueryListView.swift — removes @State isExpanded and DisclosureGroup; matches the header style of the other panels.
• ServerDashboardViewModelTests.swift (new) — Swift Testing suite covering supportedPanels per provider, kill/cancel capability matrix, and confirmation-state setters.
• docs/features/server-dashboard.mdx — drops "collapsible" wording; documents the draggable dividers and persistence.
• CHANGELOG.md — Fixed entry under [Unreleased].

Test plan

• Build in Xcode (xcodebuild -project TablePro.xcodeproj -scheme TablePro -configuration Debug build -skipPackagePluginValidation).
• Open a MySQL or PostgreSQL connection, open Server Dashboard, confirm all three panels render and Slow Queries content is visible.
• Drag each divider; confirm panels resize.
• Quit and relaunch TablePro; reopen the dashboard and confirm divider positions are restored.
• Resize the window down to the minimum (720×480); confirm Slow Queries is still reachable and the dividers stay grabbable.
• Open a SQLite connection's dashboard; confirm the one-pane layout (metrics only) still renders.
• Run the new ViewModel test suite: xcodebuild -project TablePro.xcodeproj -scheme TablePro test -skipPackagePluginValidation -only-testing:TableProTests/ServerDashboardViewModelTests.