fix(workspace): scroll sidebar instead of wrapping into columns on short viewports

[DeepDiver1975]

Fixes #146

Problem

The left nav rail is a native TabControl with TabStripPlacement="Left". Avalonia's Fluent theme gives a Left-placed tab strip a vertical WrapPanel, and since the ItemsPresenter sits directly in the template's DockPanel, a viewport too short to fit all 10 rail tabs made the rail wrap into extra side-by-side columns. At 400px height the measured columns were x=0, 172, 344, 516, 688, 860, 1032, 1204, 1376, 1548 — every tab its own column, and on a narrow window the right-most ones run off the edge entirely.

Fix

Override the TabControl control template (Styles.axaml:280) — the upstream Avalonia 12.1.1 Fluent template with exactly one change: PART_ItemsPresenter is wrapped in a ScrollViewer (which takes the DockPanel.Dock="{TemplateBinding TabStripPlacement}"" role). The WrapPanelnow measures with effectively unbounded height in its wrap direction, so the rail stays a single column and grows a scrollbar instead of wrapping. The presenter keeps itsPART_ItemsPresentername, so the theme's^[TabStripPlacement=Left] /template/ ItemsPresenter#PART_ItemsPresenter > WrapPanelrule and the app'sMargin=0` rule keep applying unchanged.

No changes to IncidentWorkspaceView.axaml, its code-behind, or any ViewModel.

Verification

• New acceptance test ModuleTabsScrollingTests.cs: renders the workspace at 1920x400 and asserts (1) all 10 tabs still present, (2) every tab shares one horizontal origin (single column), (3) the tab strip's ScrollViewer has Extent.Height > Viewport.Height (overflow, not clipping).
• Full acceptance suite: 89 passed, 0 failed (no regression in LayoutAlignmentTests gutter alignment or tab-content rendering).

Screenshots

Before (rail wrapped into per-tab columns at 400px height) and after (single scrollable column) at a short viewport: