Sort/filter dropdowns open at wrong position after scrolling

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'.

Start in src/actions/anchor.ts and inspect how the anchor position is calculated and how the popover toggle is handled. Recalculate the button position when the toggle event reports newState === 'open', then verify on /shows after scrolling that each Sort, Per Page, and Type dropdown opens directly below its button.