fix(security): declare explicit network security config to forbid cleartext

The app is purely an HTTPS API client, but the manifest left
usesCleartextTraffic implicit, which SonarQube S5332 flags (cleartext is
implicitly permitted on older Android and a merged library manifest could
re-enable it). Add an explicit network security config:

- main/release: base-config cleartextTrafficPermitted="false" (no cleartext).
- debug override (app/src/debug/res/xml): re-permits cleartext to loopback
  (127.0.0.1/localhost) only, for local dev against http://127.0.0.1:3000.

This mirrors ServerUrl's rule (HTTPS required in release, HTTP allowed in
debug via allowInsecureHttp = BuildConfig.DEBUG) at the platform socket
layer. It also fixes a latent gap: at targetSdk 28+ the platform default
already blocks cleartext, so the debug loopback path only actually works
with the explicit domain-config now added.

Docs updated in RunicGateway/docs (android/PLAN.md M1).

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

app/src/main/res/xml/network_security_config.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- SPDX-License-Identifier: GPL-3.0-or-later -->
+<!--
+  The app is purely an HTTPS API client of a shard's website backend, so the base
+  posture forbids all cleartext (HTTP) traffic. This makes explicit what minSdk 29 /
+  targetSdk 35 already default to, satisfies the "usesCleartextTraffic implicitly
+  enabled" scanner finding, and stops any merged library manifest from re-enabling
+  cleartext. It also mirrors ServerUrl's release-build rule (HTTPS required) at the
+  platform socket layer — defense in depth.
+
+  The debug variant overrides this file (app/src/debug/res/xml/) to re-permit
+  cleartext to loopback only, for local dev against http://127.0.0.1:3000.
+-->
+<network-security-config>
+    <base-config cleartextTrafficPermitted="false" />
+</network-security-config>