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/.gitignore

@@ -0,0 +1,101 @@
+# Using Android gitignore template: https://github.com/github/gitignore/blob/HEAD/Android.gitignore
+
+# Built application files
+*.apk
+*.aar
+*.ap_
+*.aab
+
+# Files for the ART/Dalvik VM
+*.dex
+
+# Java class files
+*.class
+
+# Generated files
+bin/
+gen/
+out/
+#  Uncomment the following line in case you need and you don't have the release build type files in your app
+# release/
+
+# Gradle files
+.gradle/
+build/
+
+# Local configuration file (sdk path, etc)
+local.properties
+
+# Proguard folder generated by Eclipse
+proguard/
+
+# Log Files
+*.log
+
+# Android Studio Navigation editor temp files
+.navigation/
+
+# Android Studio captures folder
+captures/
+
+# IntelliJ
+*.iml
+.idea/workspace.xml
+.idea/tasks.xml
+.idea/gradle.xml
+.idea/assetWizardSettings.xml
+.idea/dictionaries
+.idea/libraries
+# Android Studio 3 in .gitignore file.
+.idea/caches
+.idea/modules.xml
+# Comment next line if keeping position of elements in Navigation Editor is relevant for you
+.idea/navEditor.xml
+
+# Keystore files
+# Uncomment the following lines if you do not want to check your keystore files in.
+#*.jks
+#*.keystore
+
+# External native build folder generated in Android Studio 2.2 and later
+.externalNativeBuild
+.cxx/
+
+# Google Services (e.g. APIs or Firebase)
+# google-services.json
+
+# Freeline
+freeline.py
+freeline/
+freeline_project_description.json
+
+# fastlane
+fastlane/report.xml
+fastlane/Preview.html
+fastlane/screenshots
+fastlane/test_output
+fastlane/readme.md
+
+# Version control
+vcs.xml
+
+# lint
+lint/intermediates/
+lint/generated/
+lint/outputs/
+lint/tmp/
+# lint/reports/
+
+# Android Profiling
+*.hprof
+
+# Cordova plugins for Capacitor
+capacitor-cordova-android-plugins
+
+# Copied web assets
+app/src/main/assets/public
+
+# Generated Config files
+app/src/main/assets/capacitor.config.json
+app/src/main/assets/capacitor.plugins.json
+app/src/main/res/xml/config.xml