fix: sidebar tree scrolls when content overflows viewport

[Shewart]

Summary

Dogfood-driven fix (see docs/SHELLUI_DOGFOOD_FIXES.md). Any docs site with more than ~15 nav items had unreachable bottom entries — the sidebar tree grew past the viewport instead of scrolling.

The bug

.sidebar-tree had flex: 1 + overflow-y: auto but no min-height: 0. Classic flex gotcha: flex items default to min-height: auto (intrinsic content size), so flex: 1 doesn't let them shrink below the tree's content — the tree just grew past its parent's height: calc(100vh - …) container instead of scrolling.

Reproduced on:

• Desktop TopNav variant
• Desktop Sidebar (floating card) variant
• Mobile drawer

The fix

One line in src/ShellDocs.Components/Chrome/DocsSidebar.razor.css:

.sidebar-tree {
    flex: 1;
    min-height: 0;   /* NEW — lets flex shrink so overflow-y kicks in */
    …
}

All three sidebar variants route the nav tree through the same .sidebar-tree flex child, so one property fixes all of them. The existing top/bottom fade mask (mask-image: linear-gradient) still applies to the scroll viewport.

Test plan

• dotnet build shelldocs.slnx — clean, 0 warnings, 0 errors
• dotnet test shelldocs.slnx — 139 / 139 passing (CSS-only; no test surface)
• Manual: any docs site with a long nav — sidebar scrolls internally, bottom items reachable, fade mask still visible on the scroll edges

Files touched

• src/ShellDocs.Components/Chrome/DocsSidebar.razor.css — min-height: 0 on .sidebar-tree (+ one-line comment)
• docs/SHELLUI_DOGFOOD_FIXES.md — entry moved from Open to Fixed