chat theme: Set default uses SCHOOL preset's recalibrated colors

ThemeModeOverride.withFilledAppDefaults — backing the "Set default theme"
button in Chat info > Chat theme — set the wallpaper to the SCHOOL
preset but passed null for presetWallpaperTheme when filling colors. The
five-source priority chain therefore fell through to LightColorPalette /
DarkColorPalette / SimplexColorPalette / BlackColorPalette: the base
palette colors that predate this PR's preset recalibration.

Result: a chat reset via "Set default theme" showed the SCHOOL pattern
on top of pre-recalibration bubble colors that no longer match it. Pass
preset.colors[base] so the priority chain picks up SCHOOL's tuned colors
for the chosen mode, matching what the wallpaper expects.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/ui/theme/Theme.kt

@@ -661,12 +661,18 @@ data class ThemeModeOverride (
   }
 
   companion object {
-    fun withFilledAppDefaults(mode: DefaultThemeMode, base: DefaultTheme): ThemeModeOverride =
-      ThemeModeOverride(
+    fun withFilledAppDefaults(mode: DefaultThemeMode, base: DefaultTheme): ThemeModeOverride {
+      // Default override = SCHOOL preset image + SCHOOL's recalibrated colors
+      // for this base. Without passing preset.colors[base], the color chain
+      // falls through to LightColorPalette / DarkColorPalette / etc. — the
+      // pre-PR values that no longer match the recalibrated SCHOOL wallpaper.
+      val preset = PresetWallpaper.SCHOOL
+      return ThemeModeOverride(
         mode = mode,
-        colors = ThemeOverrides(base = base).withFilledColors(base, null, null, null, null, null),
-        wallpaper = ThemeWallpaper(preset = PresetWallpaper.SCHOOL.filename)
+        colors = ThemeOverrides(base = base).withFilledColors(base, null, null, null, null, preset.colors[base]),
+        wallpaper = ThemeWallpaper(preset = preset.filename)
       )
+    }
   }
 }