Never leave the desktop without a wallpaper after a theme intro (#15000)

A theme switch that plays an intro gave the shell the new still only when
the intro failed. The shell kept the old theme's path, which the theme swap
deletes, and loaded it again when OWE handed the desktop back, leaving it
empty. The new still is now handed over either way.

The shell also re-reads the background link whenever OWE hands the desktop
back or a wallpaper fails to load, falling back to the theme's first
background when the link itself is broken.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/shell.d/theme-intro-switch-test.sh

@@ -119,6 +119,9 @@ grep -Fxq "owe: intro --start first-frame --refresh $state/theme/backgrounds/int
 [[ $(readlink "$state/background") == "$state/theme/backgrounds/2-road.webp" ]] || fail "the remembered background is selected"
 ! grep -qE '^shell: background (prepare|themeTransition)' "$log" || fail "an intro replaces the normal still transition"
 [[ $(<"$home/.local/state/omarchy/background-intro.session-id") == already-consumed ]] || fail "a theme switch leaves login consumption unchanged"
+# The shell holds the old theme's still, whose path the swap deleted, and loads
+# it again when OWE hands the desktop back, so it gets the new one either way.
+wait_command "^shell: background setInstant $state/theme/backgrounds/2-road.webp\$"
 pass "switching themes plays the matching intro for the remembered background"
 
 set_theme beta
@@ -176,7 +179,12 @@ wait_command '^owe: intro '
 [[ ! -f $gates/premature-swap ]] || fail "the outgoing fade and cover settle before replacing the active theme"
 pass "rapid switches present the outgoing video cover before replacing its still"
 
-wait_command '^hypr-reload$'
+# Earlier switches finish in the background, so the next one waits for them,
+# or a late reload of theirs reads as one during its intro.
+for attempt in {1..250}; do
+  pgrep -f "^/bin/bash $ROOT/bin/omarchy-theme-set (alpha|beta)\$" >/dev/null || break
+  sleep 0.02
+done
 release="$test_tmp/reload-release"
 TEST_INTRO_PID="$test_tmp/intro.pid" TEST_RELEASE="$release" set_theme alpha
 wait_command '^owe: intro '