fix(nests): wrap NestThemedScope in Surface so LocalContentColor flips

NestThemedScope painted `themed.background` on a plain Box. Compose's
`LocalContentColor` was never updated, so it stayed at its
CompositionLocal default — `Color.Black` — for every Text inside.
On the user's dark theme that meant black-on-black for the room
title and the chat panel: unreadable.

AmethystTheme installs `MaterialTheme(colorScheme = ...)` but does
NOT wrap content in a Surface; screens that use Scaffold inherit
LocalContentColor via Scaffold's internal Surface. NestFullScreen
goes Column → verticalScroll, no Scaffold, no Surface, so the
black-default fell through.

Replace the bare `Box.background(themed.background)` with a Material
Surface that paints `color = themed.background` AND provides
`contentColor = themed.onBackground` to LocalContentColor for the
inner subtree. The `bg` image still overlays the Surface inside the
Box, exactly as before.

This was the actual cause of the user's "title impossible to read /
chat super dark" report — the event in question shipped no `c` tags
(only the unparseable `["color", "gradient-7"]`), so RoomTheme was
already Empty. The previous "all-or-nothing palette" gate (commit
15169de7) is still right for half-applied palettes; this Surface
fix is what makes empty / un-themed rooms read correctly on dark.

https://claude.ai/code/session_01RDpuki4t8StSg1CZcXnV5b

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/nests/room/NestThemedScope.kt

@@ -24,6 +24,7 @@ import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Surface
 import androidx.compose.material3.Typography
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
@@ -106,15 +107,26 @@ internal fun NestThemedScope(
         }
 
     MaterialTheme(colorScheme = themed, typography = themedTypography) {
-        // Paint the themed background COLOR on the Box itself so a room
+        // Wrap in a Surface so LocalContentColor flips to
-        // that ships `["c", hex, "background"]` but no `bg` image still
+        // `themed.onBackground` for everything inside content().
-        // visibly tints the room screen — without this, only consumers
+        // Without this, default Text() composables fall through to
-        // that explicitly read `MaterialTheme.colorScheme.background`
+        // CompositionLocal's default `Color.Black` and become
-        // would pick the override up, and the room would visually fall
+        // unreadable on dark theme — AmethystTheme installs
-        // back to the platform surface. The `bg` image, when present,
+        // MaterialTheme but doesn't itself wrap in a Surface, so any
-        // paints on top per EGG-10 rule 3 ("image overlays the color");
+        // screen that doesn't use Scaffold inherits that default.
-        // a partially-transparent image lets the color show through.
+        //
-        Box(modifier = Modifier.fillMaxSize().background(themed.background)) {
+        // Surface also paints the background color, which preserves
+        // the previous behavior (a room with `["c", hex, "background"]`
+        // visibly tints the screen). The `bg` image, when present,
+        // paints on top of Surface per EGG-10 rule 3 ("image overlays
+        // the color"); a partially-transparent image lets the color
+        // show through.
+        Surface(
+            modifier = Modifier.fillMaxSize(),
+            color = themed.background,
+            contentColor = themed.onBackground,
+        ) {
+            Box(modifier = Modifier.fillMaxSize()) {
                 theme.backgroundImageUrl?.let { url ->
                     when (theme.backgroundMode) {
                         RoomTheme.BackgroundMode.COVER -> {
@@ -135,6 +147,7 @@ internal fun NestThemedScope(
             }
         }
     }
+}
 
 /**
  * Repeating-tile background. Compose's [AsyncImage] doesn't have a