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>

API Server/internal/api/panel.go

@@ -0,0 +1,23 @@
+package api
+
+import (
+	"embed"
+	"io/fs"
+	"net/http"
+)
+
+// The gsmnode web panel: a Vue 3 + Tailwind app (source in panel/, built
+// with `npm run build` into dist/) embedded at compile time and served at
+// the server root.
+//
+//go:embed all:dist
+var panelFS embed.FS
+
+// panelHandler serves the built panel assets.
+func panelHandler() http.Handler {
+	sub, err := fs.Sub(panelFS, "dist")
+	if err != nil {
+		panic(err) // embedded dist is malformed; unreachable in a valid build
+	}
+	return http.FileServerFS(sub)
+}