build: remove expo.dev/EAS dependency, add local Android build pipeline

- Delete eas.json and strip extra.eas.projectId from app.json
- Add plugins/withAndroidReleaseSigning.js — Expo config plugin that injects
  release signingConfig into the generated build.gradle during expo prebuild
- Add build-apk.sh — self-contained build script (expo prebuild + Gradle)
  Reads keystore credentials from ~/.config/postiz-mobile/signing.env
  Outputs APK/AAB to dist/, wipes credentials from gradle.properties after build
- Add install-android-sdk.sh — one-time Android SDK cmdline-tools bootstrap
- Remove unused expo-task-manager dependency
- Update .gitignore: android/, ios/, static-build/ excluded (generated)

Build workflow:
  1. eas credentials --platform android  # export keystore once
  2. ./install-android-sdk.sh            # first time only
  3. ./build-apk.sh                      # → dist/postiz-mobile-YYYYMMDD-HHMM.apk

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

artifacts/postiz-mobile/install-android-sdk.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+# install-android-sdk.sh — Install Android SDK command-line tools only (no Android Studio).
+# Installs to ~/android-sdk. Run once; takes ~1 GB of disk space.
+
+set -euo pipefail
+
+SDK_DIR="$HOME/android-sdk"
+CMDLINE_TOOLS_URL="https://dl.google.com/android/repository/commandlinetools-linux-12266719_latest.zip"
+CMDLINE_ZIP="/tmp/android-cmdline-tools.zip"
+
+GREEN='\033[0;32m'; NC='\033[0m'
+info() { echo -e "${GREEN}[sdk-install]${NC} $*"; }
+
+if [ -d "$SDK_DIR/cmdline-tools/latest/bin" ]; then
+  info "Android SDK already installed at $SDK_DIR"
+  exit 0
+fi
+
+info "Downloading Android command-line tools…"
+wget -q --show-progress -O "$CMDLINE_ZIP" "$CMDLINE_TOOLS_URL"
+
+info "Extracting…"
+mkdir -p "$SDK_DIR/cmdline-tools"
+unzip -q "$CMDLINE_ZIP" -d "$SDK_DIR/cmdline-tools"
+mv "$SDK_DIR/cmdline-tools/cmdline-tools" "$SDK_DIR/cmdline-tools/latest"
+rm "$CMDLINE_ZIP"
+
+export ANDROID_HOME="$SDK_DIR"
+export PATH="$PATH:$SDK_DIR/cmdline-tools/latest/bin:$SDK_DIR/platform-tools"
+
+info "Accepting licenses…"
+yes | sdkmanager --licenses > /dev/null 2>&1 || true
+
+info "Installing SDK components (platform-tools, build-tools 35, NDK 28)…"
+sdkmanager \
+  "platform-tools" \
+  "platforms;android-35" \
+  "build-tools;35.0.0" \
+  "ndk;28.0.12433566"
+
+info "Done. Add this to your ~/.bashrc or ~/.zshrc:"
+echo ""
+echo '  export ANDROID_HOME="$HOME/android-sdk"'
+echo '  export PATH="$PATH:$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools"'
+echo ""
+info "Then reload your shell: source ~/.bashrc"