Add Flutter dev container configuration for mobile development

Adds a dedicated dev container under .devcontainer/mobile/ following the
monorepo multi-container pattern. Includes a multi-stage Dockerfile with
Android SDK (platform 36) and Flutter 3.29.2, a devcontainer.json with
Dart/Flutter VS Code extensions, and an on-create script that configures
Flutter for Android-only development and installs project dependencies.

https://claude.ai/code/session_013F3TXesccdf2ta5qwykPtt

.devcontainer/mobile/Dockerfile

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+# Multi-stage Dockerfile for Flutter Android development
+# Inspired by: https://github.com/wtfzambo/flutter-devcontainer-template
+
+# --- Builder Stage ---
+FROM --platform=linux/amd64 debian:bookworm-slim AS builder
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+ENV ANDROID_SDK_ROOT=/android-sdk
+ENV FLUTTER_SDK_ROOT=/flutter
+
+RUN apt-get update \
+  && apt-get -y install --no-install-recommends \
+    wget \
+    unzip \
+    xz-utils \
+    openjdk-17-jdk \
+  && apt-get autoremove -y \
+  && apt-get clean -y \
+  && rm -rf /var/lib/apt/lists/*
+
+# Download and install Android Command Line Tools
+RUN mkdir -p ${ANDROID_SDK_ROOT}/cmdline-tools \
+  && wget -O commandlinetools.zip "https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip" \
+  && unzip commandlinetools.zip -d ${ANDROID_SDK_ROOT}/cmdline-tools \
+  && mv ${ANDROID_SDK_ROOT}/cmdline-tools/cmdline-tools ${ANDROID_SDK_ROOT}/cmdline-tools/latest \
+  && rm commandlinetools.zip
+
+# Accept Android SDK licenses and install necessary components
+# compileSdk=36, minSdkVersion=24 per mobile/android/app/build.gradle
+RUN yes | ${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin/sdkmanager --licenses && \
+  ${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin/sdkmanager \
+    "build-tools;34.0.0" \
+    "platform-tools" \
+    "platforms;android-36"
+
+# Download and install Flutter (>=3.27.0 per mobile/pubspec.yaml)
+RUN mkdir -p ${FLUTTER_SDK_ROOT} \
+  && wget -O flutter.tar.xz "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.29.2-stable.tar.xz" \
+  && tar xf flutter.tar.xz -C ${FLUTTER_SDK_ROOT} --strip-components=1 \
+  && rm flutter.tar.xz
+
+# --- Final Stage ---
+FROM --platform=linux/amd64 debian:bookworm-slim AS final
+
+ARG USERNAME=vscode
+ARG USER_UID=1000
+ARG USER_GID=$USER_UID
+ARG HOME=/home/${USERNAME}
+
+ENV ANDROID_SDK_ROOT=${HOME}/android-sdk
+ENV FLUTTER_SDK_ROOT=${HOME}/flutter
+ENV PATH=$PATH:${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin
+ENV PATH=$PATH:${ANDROID_SDK_ROOT}/platform-tools
+ENV PATH=$PATH:${FLUTTER_SDK_ROOT}/bin
+
+RUN dpkg --add-architecture amd64 \
+  && apt-get update \
+  && apt-get -y install --no-install-recommends \
+    curl \
+    git \
+    unzip \
+    xz-utils \
+    zip \
+    openjdk-17-jdk \
+    libglu1-mesa \
+    libc6:amd64 \
+    sudo \
+  && apt-get autoremove -y \
+  && apt-get clean -y \
+  && rm -rf /var/lib/apt/lists/*
+
+# Create a non-root user
+RUN groupadd --gid ${USER_GID} ${USERNAME} \
+  && useradd --uid ${USER_UID} --gid ${USER_GID} -m ${USERNAME} \
+  && echo ${USERNAME} ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/${USERNAME} \
+  && chmod 0440 /etc/sudoers.d/${USERNAME}
+
+USER ${USERNAME}
+
+# Copy SDKs from the builder stage to the user's home directory
+COPY --from=builder --chown=${USERNAME}:${USERNAME} /android-sdk ${ANDROID_SDK_ROOT}
+COPY --from=builder --chown=${USERNAME}:${USERNAME} /flutter ${FLUTTER_SDK_ROOT}

.devcontainer/mobile/devcontainer.json

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+{
+  "name": "Sure Mobile (Flutter)",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "args": {
+      "USERNAME": "${localEnv:USER}${localEnv:USERNAME}"
+    }
+  },
+  "containerEnv": {
+    "SHELL": "/bin/bash"
+  },
+  "runArgs": [
+    "--platform",
+    "linux/amd64",
+    "--name",
+    "${localWorkspaceFolderBasename}_flutter_devcontainer"
+  ],
+  "onCreateCommand": "bash ./.devcontainer/mobile/on-create.sh",
+  "remoteUser": "${localEnv:USER}${localEnv:USERNAME}",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "Dart-Code.dart-code",
+        "Dart-Code.flutter"
+      ],
+      "settings": {
+        "terminal.integrated.defaultProfile.linux": "bash",
+        "dart.flutterSdkPath": "/home/${localEnv:USER}${localEnv:USERNAME}/flutter"
+      }
+    }
+  }
+}

.devcontainer/mobile/on-create.sh

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+#!/bin/bash
+set -e
+
+# Disable analytics for CI/container environments
+dart --disable-analytics
+flutter --disable-analytics
+
+# Configure Flutter for Android-only development
+flutter config --no-enable-web
+flutter config --no-enable-linux-desktop
+flutter config --no-enable-macos-desktop
+flutter config --no-enable-windows-desktop
+
+# Verify the development environment
+flutter doctor
+
+# Install project dependencies
+cd mobile
+flutter pub get