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>

Web App/server/dist/gsmnode-mark.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="40" height="40" viewBox="0 0 40 40" fill="none">
+  <line x1="7" y1="15" x2="31" y2="15" stroke="#12161C" stroke-width="2.6" stroke-linecap="round"></line>
+  <path d="M26 10 L32 15 L26 20" stroke="#12161C" stroke-width="2.6" stroke-linecap="round" stroke-linejoin="round"></path>
+  <line x1="9" y1="25" x2="33" y2="25" stroke="#2E9E6B" stroke-width="2.6" stroke-linecap="round"></line>
+  <path d="M14 20 L8 25 L14 30" stroke="#2E9E6B" stroke-width="2.6" stroke-linecap="round" stroke-linejoin="round"></path>
+</svg>