feat(#11): make HermesScaffold title a @Composable slot, convert ChatScreen (#12)

- HermesScaffold: title now @Composable () -> Unit (M3-idiomatic)
- HermesScaffold: added modifier and snackbarHost parameters
- Updated all 18 callers from title="X" to title={Text("X")}
- ChatScreen: converted from raw Scaffold to HermesScaffold
- Removed unused Menu icon import (ktlint fix)

app/src/main/java/com/m57/hermescontrol/ui/plugins/PluginsScreen.kt

@@ -57,7 +57,7 @@ fun PluginsScreen(
     }
 
     HermesScaffold(
-        title = "Plugins",
+        title = { Text("Plugins") },
         onOpenDrawer = onOpenDrawer,
         onRefresh = { viewModel.loadPlugins() },
     ) { paddingValues ->