Sort/filter dropdowns open at wrong position after scrolling

[2u841r]

Bug

The Sort, Per Page, and Type filter dropdowns on the /shows page open at the wrong
screen position if the user has scrolled down before clicking them.

Steps to Reproduce

1. Go to /shows
2. Scroll down the page
3. Click any of the filter buttons (Type, Per Page, Sort)

Expected

Dropdown opens directly below the button that was clicked.

Actual

Dropdown appears offset — anchored to where the button was when the page first loaded,
not its current viewport position.

Root Cause

src/actions/anchor.ts calculates button position via getBoundingClientRect() but
only recalculates on resize and ResizeObserver — never when the popover opens.
After scrolling, the cached position is stale.

Fix

Listen for the popover toggle event and recalculate position when newState === 'open'.