fix(#3548): pin secondary menu at bottom of Work Side Menu during scroll

[twjeffery]

Summary

• Restructures the Work Side Menu so only the primary nav area scrolls, while the secondary menu (Search, Get support, Release notes), account section, and collapse toggle stay pinned at the bottom
• Adds scroll-aware border and shadow indicators that fade in/out (300ms) based on scroll position, only showing when there is hidden content in that direction
• Adds browser test verifying secondary menu remains visible when primary nav overflows

Screen.Recording.2026-03-30.at.3.48.30.PM.mov

Top of scroll

### Middle of scroll ### Bottom of scroll ### No scroll

Closes #3548

Test plan

• Open the PR playground at /bugs/3548 which has enough primary items to overflow
• Verify primary nav scrolls independently while secondary menu stays pinned at bottom
• Verify header stays fixed at top, items don't scroll behind it
• Verify border and shadow appear/fade when scrolled away from top or bottom
• Verify border and shadow disappear when scrolled fully to either end
• Collapse the menu and verify layout still works at 72px width
• Resize the viewport height and confirm the layout adapts
• Test on mobile viewport (menu opens as full-screen modal)