Fix desktop theme contrast: missing on* color roles

lightColorScheme()/darkColorScheme() only overrode primary/secondary/
tertiary/*Container/surface, leaving every paired on* text/icon role at
Material3's baseline defaults — tuned for the default purple palette,
with no guaranteed contrast against Amber's custom light gold/cream
colors. This made default-styled Button() content (Copy to clipboard,
Add relay, theme toggle, Revoke all permissions, Always allow) and
NavigationRail's selected-item labels nearly invisible.

Adds explicit onPrimary/onSecondary/onTertiary/onPrimaryContainer/
onSecondaryContainer/onSurface/onSurfaceVariant/onBackground/outline
using a dark warm "ink" tone, fixing every default-styled button in one
place rather than per call site. Also fixes the dark scheme's onPrimary
(was White, same bug — primary stays light gold in dark mode too).

HomeScreen's relay-status text and ActivityScreen's approved/rejected
badge now use shared positiveColor/negativeColor instead of raw
colorScheme.primary (itself too light to read as body text) or a
locally-duplicated hardcoded green/red. Divider/border colors that used
colorScheme.primary switch to the new colorScheme.outline role.

desktop/src/main/kotlin/com/greenart7c3/nostrsigner/desktop/ui/ActivityScreen.kt

@@ -22,6 +22,8 @@ import com.greenart7c3.nostrsigner.desktop.ui.components.LetterAvatar
 import com.greenart7c3.nostrsigner.desktop.ui.components.bunkerMethodDescription
 import com.greenart7c3.nostrsigner.desktop.ui.components.relativeTimeFromNow
 import com.greenart7c3.nostrsigner.desktop.ui.components.shortenHex
+import com.greenart7c3.nostrsigner.desktop.ui.theme.negativeColor
+import com.greenart7c3.nostrsigner.desktop.ui.theme.positiveColor
 
 @Composable
 fun ActivityScreen(history: List<HistoryRow>) {
@@ -65,7 +67,7 @@ fun ActivityScreen(history: List<HistoryRow>) {
 
 @Composable
 private fun ApprovedBadge(approved: Boolean) {
-    val color = if (approved) Color(0xFF2E7D32) else Color(0xFFC62828)
+    val color = if (approved) positiveColor else negativeColor
     Surface(color = color, shape = RoundedCornerShape(4.dp)) {
         Text(
             if (approved) "Allowed" else "Rejected",

desktop/src/main/kotlin/com/greenart7c3/nostrsigner/desktop/ui/AppDetailScreen.kt

@@ -112,7 +112,7 @@ private fun PermissionRow(
         modifier = Modifier
             .fillMaxWidth()
             .padding(vertical = 4.dp)
-            .border(BorderStroke(1.dp, MaterialTheme.colorScheme.primary), RoundedCornerShape(6.dp))
+            .border(BorderStroke(1.dp, MaterialTheme.colorScheme.outline), RoundedCornerShape(6.dp))
             .padding(12.dp),
     ) {
         Text(bunkerMethodDescription(permission.method, permission.kind), style = MaterialTheme.typography.bodyLarge)

desktop/src/main/kotlin/com/greenart7c3/nostrsigner/desktop/ui/ConnectedAppsScreen.kt

@@ -51,7 +51,7 @@ fun ConnectedAppsScreen(connectedApps: List<ConnectedApp>, onAppClick: (String)
                         Text("Last used ${relativeTimeFromNow(app.connectedAt)}", style = MaterialTheme.typography.bodySmall)
                     }
                 }
-                HorizontalDivider(color = MaterialTheme.colorScheme.primary)
+                HorizontalDivider(color = MaterialTheme.colorScheme.outline)
             }
         }
     }

desktop/src/main/kotlin/com/greenart7c3/nostrsigner/desktop/ui/HomeScreen.kt

@@ -20,6 +20,8 @@ import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import com.greenart7c3.nostrsigner.desktop.ui.components.LetterAvatar
 import com.greenart7c3.nostrsigner.desktop.ui.components.shortenHex
+import com.greenart7c3.nostrsigner.desktop.ui.theme.negativeColor
+import com.greenart7c3.nostrsigner.desktop.ui.theme.positiveColor
 
 @Composable
 fun HomeScreen(
@@ -52,7 +54,7 @@ fun HomeScreen(
                 "Connected to $connectedRelayCount of $totalRelays relays"
             },
             style = MaterialTheme.typography.bodySmall,
-            color = if (connectedRelayCount > 0) MaterialTheme.colorScheme.primary else MaterialTheme.colorScheme.error,
+            color = if (connectedRelayCount > 0) positiveColor else negativeColor,
         )
 
         Spacer(Modifier.height(24.dp))

desktop/src/main/kotlin/com/greenart7c3/nostrsigner/desktop/ui/theme/DesktopTheme.kt

@@ -15,23 +15,51 @@ val primaryVariant = Color(0xFFC8541A)
 val secondaryColor = Color(0xFFFFCA62)
 val orange = Color(0xFFFF6B00)
 
+/**
+ * `primary`/`tertiary`/`primaryContainer`/`secondaryContainer` are all a light gold in both
+ * schemes, so their paired `on*` roles need an explicit dark color — Material3's baseline
+ * defaults (tuned for the default purple palette) don't have guaranteed contrast against a
+ * custom light container and were left unset here before, making button/nav-rail text
+ * unreadable.
+ */
+val inkColor = Color(0xFF3E2A0D)
+val outlineColor = Color(0xFF8A6D3A)
+val positiveColor = Color(0xFF2E7D32)
+val negativeColor = Color(0xFFC62828)
+
 private val DarkColorPalette = darkColorScheme(
     primary = primaryColor,
-    onPrimary = Color.White,
+    onPrimary = inkColor,
     secondary = primaryVariant,
+    onSecondary = Color.White,
     tertiary = secondaryColor,
+    onTertiary = inkColor,
     primaryContainer = secondaryColor,
+    onPrimaryContainer = inkColor,
     secondaryContainer = secondaryColor,
+    onSecondaryContainer = inkColor,
 )
 
+private val surfaceColor = Color(0xFFFFDE9E)
+
 private val LightColorPalette = lightColorScheme(
     primary = primaryColor,
+    onPrimary = inkColor,
     secondary = primaryVariant,
+    onSecondary = Color.White,
     tertiary = secondaryColor,
+    onTertiary = inkColor,
     primaryContainer = secondaryColor,
+    onPrimaryContainer = inkColor,
     secondaryContainer = secondaryColor,
-    surface = Color(0xFFFFDE9E),
-    surfaceContainer = Color(0xFFFFDE9E),
+    onSecondaryContainer = inkColor,
+    surface = surfaceColor,
+    surfaceContainer = surfaceColor,
+    onSurface = inkColor,
+    onSurfaceVariant = inkColor,
+    background = surfaceColor,
+    onBackground = inkColor,
+    outline = outlineColor,
 )
 
 val DesktopShapes = Shapes(