Load one picture at a time on a Guition: an alert's picture before a media tile's cover (0.2.77)

A download shares ESPHome's loop with touch and drawing. An alert already
closes every card, and the card's cover with it; a media tile over the whole
page stays under the alert. Its cover now waits while the alert's picture is
announced, loading or due for a retry, and an alert picture that arrives while
a cover is on its way (or a dropped cover's download is still being closed)
starts right after it. The host harness checks all three cases on the Guition.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

screen_manager/CHANGELOG.md

@@ -5,6 +5,7 @@ A media card like a phone's "now playing", with the album cover on a Guition.
 - **The media card is new.** Tap a media player's tile and the card shows the album cover, the title, the artist and the album, a progress bar that runs while the track plays with the elapsed and total time at its ends, three round keys (previous, play or pause on the blue key, next) and a volume row: a mute key, a wide slider with a round knob, and the percentage. The keys the player does not offer are faded, the mute key and the slider follow what Home Assistant reports (a muted player shows a grey fill and *Muted*), and a player that is off shows one power key. A stream without a length shows no bar and no times. It works in the light and the dark look.
 - **The album cover** comes the way camera images do (app 0.2.66): the screen asks ESP Screen Manager, the app fetches the picture from Home Assistant with its own access, makes it exactly the size the card asks for with the corners already rounded over the card's colour, and serves it on port 8098. A Guition loads it once and keeps it until the picture changes, the card closes or the page turns. The CYD has no memory for pictures and shows the player's icon in the cover's place, as it does when a player has no picture (a radio station, a player that is off).
 - **A media tile over the whole page** (size *Full page*) is the same card under the tile's head: the cover at the left, the track, the bar and the keys beside it, the volume row along the bottom. The keys and the slider work on the tile; holding the tile still opens the card.
+- **One picture at a time.** A download shares the screen's loop with touch and drawing, so a Guition never loads two pictures at once. An alert closes an open card, and the card's cover goes with it. A media tile over the whole page stays under the alert, and there the alert's picture goes first: a cover that has not started waits until the alert's picture is there, and a cover already on its way finishes before the alert's picture starts. A camera full screen and the cover share one picture, so they never load together. Memory is not the limit: the pictures live in PSRAM (a cover about 70 KB, the alert's 172 KB).
 - **The card's clock runs on the screen.** Home Assistant reports where a track was and when; the bar and the elapsed time run on from there every second without any traffic, and stand still while the track is paused.
 - Claude in Home Assistant and the editor need nothing new: a `media_player` tile is the same tile, with the new card behind it.
 - Needs firmware 0.2.64 for the new card; the app serves covers only to a Guition on that firmware. Older firmware keeps the old card. Includes everything from 0.2.76.