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/respond.go

@@ -0,0 +1,65 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"log"
+	"net/http"
+
+	"smsgateway/apiserver/internal/pb"
+)
+
+// writeJSON writes v as a JSON response with the given status code.
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if v == nil {
+		return
+	}
+	if err := json.NewEncoder(w).Encode(v); err != nil {
+		log.Printf("writeJSON: %v", err)
+	}
+}
+
+// errorBody is the standard error envelope.
+type errorBody struct {
+	Error string `json:"error"`
+}
+
+// writeError writes a JSON error response.
+func writeError(w http.ResponseWriter, status int, msg string) {
+	writeJSON(w, status, errorBody{Error: msg})
+}
+
+// writeUpstreamError maps a PocketBase error onto an appropriate HTTP status.
+func writeUpstreamError(w http.ResponseWriter, err error) {
+	var apiErr *pb.APIError
+	if errors.As(err, &apiErr) {
+		switch apiErr.Status {
+		case http.StatusNotFound:
+			writeError(w, http.StatusNotFound, "not found")
+		case http.StatusBadRequest:
+			writeError(w, http.StatusBadRequest, apiErr.Message)
+		case http.StatusForbidden:
+			writeError(w, http.StatusForbidden, "forbidden")
+		default:
+			log.Printf("upstream error: %v (%s)", apiErr, apiErr.Body)
+			writeError(w, http.StatusBadGateway, "upstream error")
+		}
+		return
+	}
+	log.Printf("internal error: %v", err)
+	writeError(w, http.StatusInternalServerError, "internal error")
+}
+
+// decodeJSON reads and decodes a JSON request body into v.
+func decodeJSON(r *http.Request, v any) error {
+	defer io.Copy(io.Discard, r.Body)
+	dec := json.NewDecoder(http.MaxBytesReader(nil, r.Body, 1<<20))
+	dec.DisallowUnknownFields()
+	if err := dec.Decode(v); err != nil {
+		return err
+	}
+	return nil
+}