WIP - Add installer downloader and in-app upgrades backend

.github/actions/check-file-size/action.yml

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+name: "Check file size"
+description: "Fails a file exceeds a given size limit"
+inputs:
+  artifact:
+    description: "Path to the file"
+    required: true
+  max_size:
+    description: "Maximum allowed size in bytes"
+    required: true
+runs:
+  using: "composite"
+  steps:
+    - name: Check file size
+      shell: bash
+      run: |
+        if [ -f "${{ inputs.artifact }}" ]; then
+          SIZE=$(stat -c %s "${{ inputs.artifact }}")
+          echo "File size: $SIZE bytes"
+          echo "Size limit: ${{ inputs.max_size }} bytes"
+
+          if [ "$SIZE" -gt "${{ inputs.max_size }}" ]; then
+            echo "Error: Binary size exceeds limit."
+            exit 1
+          fi
+        else
+          echo "Error: File not found!"
+          exit 1
+        fi

.github/workflows/downloader.yml

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+---
+name: Installer downloader - Size test
+on:
+  pull_request:
+    paths:
+      - '**'
+      - '!**/**.md'
+      - '!.github/workflows/**'
+      - '.github/workflows/downloader.yml'
+      - '!.github/CODEOWNERS'
+      - '!android/**'
+      - '!audits/**'
+      - '!build.sh'
+      - '!ci/**'
+      - '!clippy.toml'
+      - '!deny.toml'
+      - '!rustfmt.toml'
+      - '!.yamllint'
+      - '!docs/**'
+      - '!graphics/**'
+      - '!desktop/**'
+      - '!ios/**'
+      - '!scripts/**'
+      - '!.*ignore'
+      - '!prepare-release.sh'
+      - '!**/osv-scanner.toml'
+
+permissions: {}
+
+jobs:
+  build-windows:
+    strategy:
+      matrix:
+        config:
+          - os: windows-latest
+            arch: x64
+    runs-on: ${{ matrix.config.os }}
+    env:
+      # If the file is larger than this, a regression has probably been introduced.
+      # You should think twice before increasing this limit.
+      MAX_BINARY_SIZE: 1572864
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Build
+        shell: bash
+        env:
+          # On Windows, the checkout is on the D drive, which is very small.
+          # Moving the target directory to the C drive ensures that the runner
+          # doesn't run out of space on the D drive.
+          CARGO_TARGET_DIR: "C:/cargo-target"
+        run: ./installer-downloader/build.sh
+
+      - name: Check file size
+        uses: ./.github/actions/check-file-size
+        with:
+          artifact: "C:/cargo-target/release/installer-downloader.exe"
+          max_size: ${{ env.MAX_BINARY_SIZE }}
+
+  build-macos:
+    runs-on: macos-latest
+    env:
+      # TODO: Figure out a reasonable limit for macOS
+      # If the file is larger than this, a regression has probably been introduced.
+      # You should think twice before increasing this limit.
+      MAX_BINARY_SIZE: 2097152
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Build
+        shell: bash
+        run: ./installer-downloader/build.sh
+
+      - name: Check file size
+        uses: ./.github/actions/check-file-size
+        with:
+          artifact: "./target/release/installer-downloader"
+          max_size: ${{ env.MAX_BINARY_SIZE }}