fix(#728): resolve theme color leakage for agent chat bubbles (#730)

Use MaterialTheme.colorScheme tokens for Assistant, System, and Tool bubbles so that all presets and dynamic colors render properly without theme bleed.

app/src/main/java/com/m57/hermescontrol/theme/Color.kt

@@ -75,7 +75,7 @@ val StatusGrey = Color(0xFF9E9E9E)
 val StatusGreyDark = Color(0xFF1A1A24)
 val StatusGreyLight = Color(0xFFF5F5F5)
 
-// ── Chat-specific colours (always brand-defined for consistency) ───────
+// ── Chat-specific colours (default theme fallback tokens) ─────────────────
 
 val AssistantBubble = Color(0xFF1C1C26)
 val AssistantBubbleLight = Color(0xFFEDEAF4)