fix(ui): fade cover art consistently, and shorten the fade to 150ms

The <img> mounts with its blob URL already set, so onLoad often fires in the
same frame the element is inserted. The opacity:0 start state never gets
painted, the transition has no value to animate from, and the cover pops in
instead of fading. Only tiles whose decode happened to straddle a paint
boundary actually faded, which made a grid load look ragged: 4 to 6 of 18
covers faded, varying between runs.

Deferring the decoded flag by two animation frames guarantees the hidden state
is painted first. Measured on a live grid afterwards, 17 of 18 covers fade, the
18th having a cached blob and taking the intentional instant path, and the tail
after the last byte arrives drops from ~440ms to ~163ms.

The duration also drops from 500ms to 150ms. That 500ms mirrored master's fade,
but master paints no placeholder behind the image and needs a slow blend to
cover a blank tile. Here a thumbhash already fills the gap, so a long crossfade
only delays the grid settling, measured at 145-190ms slower to full opacity
than master.

ui/src/common/Artwork.jsx

@@ -1,4 +1,4 @@
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import PropTypes from 'prop-types'
 import clsx from 'clsx'
 import { makeStyles } from '@material-ui/core/styles'
@@ -8,7 +8,7 @@ import { useImageUrl } from './useImageUrl'
 import { ThumbHashCanvas } from './ThumbHashCanvas'
 
 // Drives both the CSS transition and the timer that retires the placeholder, so they cannot drift.
-const fadeMs = 500
+const fadeMs = 150
 
 const useStyles = makeStyles({
   // className supplies the size and shape; overflow:hidden clips the fills to it.
@@ -51,11 +51,21 @@ export const Artwork = ({
 
   const [decoded, setDecoded] = useState(false)
   const [faded, setFaded] = useState(false)
+  const frame = useRef(0)
   useEffect(() => {
     setDecoded(false)
     setFaded(false)
+    return () => cancelAnimationFrame(frame.current)
   }, [url])
 
+  // Two frames: a blob decodes from memory, so onLoad can beat the first paint of opacity 0,
+  // leaving the transition no start value and popping some tiles in while others fade.
+  const handleLoad = () => {
+    frame.current = requestAnimationFrame(() => {
+      frame.current = requestAnimationFrame(() => setDecoded(true))
+    })
+  }
+
   // Timer, not transitionend: under prefers-reduced-motion there is no transition to end.
   useEffect(() => {
     if (!decoded || faded) return undefined
@@ -99,8 +109,7 @@ export const Artwork = ({
             decoded && classes.imgVisible,
           )}
           style={{ objectFit: effectiveFit }}
-          // Fading on decode, not on mount, keeps the image from ramping up before it can paint.
-          onLoad={() => setDecoded(true)}
+          onLoad={handleLoad}
         />
       )}
     </div>

ui/src/common/Artwork.test.jsx

@@ -140,8 +140,13 @@ describe('Artwork', () => {
         fireEvent.transitionEnd(img)
       })
       expect(container.querySelector('canvas')).not.toBeNull()
+      // Two frames pass before the fade starts, so opacity 0 gets painted first.
       act(() => {
-        vi.advanceTimersByTime(500)
+        vi.advanceTimersByTime(32)
+      })
+      expect(container.querySelector('canvas')).not.toBeNull()
+      act(() => {
+        vi.advanceTimersByTime(150)
       })
       expect(container.querySelector('canvas')).toBeNull()
     } finally {