fix(concord): make the invite failure messages visible in the dark theme

Found while proving the new ban gate on a tablet: the refusal reached the
view hierarchy but rendered as black pixels on the black background, so
the screen looked blank and the user was told nothing.

`ConcordInviteScreen`'s Column sits on the bare window background with no
Surface above it, so `LocalContentColor` is still Material 3's default
black. This predates the invite work and silently affects every state the
screen can end in — invalid, incompatible, revoked, expired, unreachable —
plus the "Redeeming invite…" progress label, which is why only the spinner
was ever visible while a join was in flight.

Verified on device: the message now renders.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/publicChannels/concord/ConcordInviteScreen.kt

@@ -163,6 +163,10 @@ fun ConcordInviteScreen(
                 Text(
                     stringRes(R.string.concord_redeeming_invite),
                     modifier = Modifier.padding(top = 16.dp),
+                    // Explicit: this Column sits on the bare window background with no Surface
+                    // above it, so LocalContentColor is still the M3 default black — which renders
+                    // every one of these labels invisible in the dark theme.
+                    color = MaterialTheme.colorScheme.onBackground,
                     textAlign = TextAlign.Center,
                 )
             }
@@ -172,6 +176,7 @@ fun ConcordInviteScreen(
                 Text(
                     stringRes(failed.messageRes),
                     style = MaterialTheme.typography.bodyLarge,
+                    color = MaterialTheme.colorScheme.onBackground,
                     textAlign = TextAlign.Center,
                 )
                 if (failed.canRetry) {