feat(mobile): Android app shell with Capacitor

The web client already has a full mobile UI (MobileShell and 17 sibling
screens) and ships as a PWA, so what was missing to get an APK was a native
container, not an interface.

The WebView loads the instance over the network instead of bundling a copy of
the web build. Bundling would mean republishing an APK on every site deploy —
there is no electron-updater equivalent here — and it would move the page to
https://localhost, turning every same-origin /api and /ws call into a
cross-origin one. The cost is no offline mode, which for a realtime chat app
costs nothing: public/index.html covers the unreachable-server case.

packages/mobile sits outside the pnpm workspace on purpose. Capacitor's CLI and
Gradle both assume a flat node_modules, and keeping it out leaves pnpm-lock.yaml
and the existing web/desktop CI untouched by it.

Icons are generated from the existing brand art. The launch screen is a
layer-list over the brand background rather than Capacitor's per-density
splash.png set, which is light and flashed white before the WebView painted.

CI builds an *unsigned* APK. The signing key stays on the VPS and is applied
there — an APK signature is what proves an update came from us, so the key has
no reason to exist on a second machine.

Background voice (foreground service) is not in this commit: a plain WebView is
throttled when the screen goes off and the call drops. That is the next phase.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/mobile/android/app/proguard-rules.pro

@@ -0,0 +1,21 @@
+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# If your project uses WebView with JS, uncomment the following
+# and specify the fully qualified class name to the JavaScript interface
+# class:
+#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
+#   public *;
+#}
+
+# Uncomment this to preserve the line number information for
+# debugging stack traces.
+#-keepattributes SourceFile,LineNumberTable
+
+# If you keep the line number information, uncomment this to
+# hide the original source file name.
+#-renamesourcefileattribute SourceFile