security: restrict network to system CAs, tighten WebView capabilities; v1.9

- AndroidManifest: add networkSecurityConfig to explicitly trust only system
  CAs, preventing user-installed CA cert MITM attacks on claude.ai sessions
- LoginActivity: set javaScriptCanOpenWindowsAutomatically=false (not needed
  for claude.ai login) and databaseEnabled=false (deprecated WebSQL)
- build.gradle.kts: enable buildConfig generation (required for
  BuildConfig.DEBUG guards already used in UsageRepository)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<network-security-config>
+    <base-config cleartextTrafficPermitted="false">
+        <trust-anchors>
+            <certificates src="system" />
+        </trust-anchors>
+    </base-config>
+</network-security-config>