Rebrand CommGate to gsmnode and adopt new design system

Rename the whole project from CommGate to gsmnode and re-skin every
surface onto the new signal-green + ink design system (Space Grotesk /
IBM Plex Sans / JetBrains Mono, flat surfaces, two-arrow routing mark,
lowercase gsm+node wordmark).

- Web App + API panel: rewrite token layer, gn-* utilities, data-gsm-theme,
  new logo/favicon assets; both builds verified.
- Phone App (Flutter): green theme, new mark/wordmark widgets, adaptive
  launcher icons; rename Android applicationId/package to app.gsmnode.phone;
  bump AGP to 8.6.0 and google_fonts to 8.1.0; debug APK build verified.
- API Server (Go): panel routes, User-Agent, health service id, branding.
- Home Assistant Plugin: rename integration domain sms_gateway to gsmnode
  (folder, DOMAIN, services, entity ids, classes); py_compile verified.
- Update all READMEs; add .gitignore (excludes Design/, build artifacts).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Phone App/lib/models/message.dart

@@ -0,0 +1,33 @@
+/// An outbound message handed to the device by the API Server.
+class GatewayMessage {
+  final String id;
+  final String type; // 'sms' or 'call'
+  final List<String> phoneNumbers;
+  final String textMessage;
+  final int? simNumber;
+  final String status;
+
+  GatewayMessage({
+    required this.id,
+    required this.type,
+    required this.phoneNumbers,
+    required this.textMessage,
+    this.simNumber,
+    required this.status,
+  });
+
+  bool get isCall => type == 'call';
+
+  factory GatewayMessage.fromJson(Map<String, dynamic> json) {
+    return GatewayMessage(
+      id: json['id'] as String? ?? '',
+      type: json['type'] as String? ?? 'sms',
+      phoneNumbers: (json['phone_numbers'] as List<dynamic>? ?? [])
+          .map((e) => e.toString())
+          .toList(),
+      textMessage: json['text_message'] as String? ?? '',
+      simNumber: json['sim_number'] as int?,
+      status: json['status'] as String? ?? '',
+    );
+  }
+}