feat(client): implement Windows client (Phase 2)

- Persistent shell session (cmd.exe) preserving cd state between commands
- Screenshot capture via Windows GDI (BGRA→RGBA→PNG→Base64)
- Mouse click via SendInput with absolute screen coordinates
- Text input via SendInput with Unicode (UTF-16) key events
- Auto-reconnect with exponential backoff (max 30s)
- Config stored in %APPDATA%/helios-remote/config.json
- All Windows APIs under #[cfg(windows)] for cross-compile safety
- CI: add Windows cross-compile job (x86_64-pc-windows-gnu) with artifact upload

.github/workflows/ci.yml

@@ -3,6 +3,7 @@ name: CI
 on:
   push:
     branches: ["main", "master"]
+    tags: ["v*"]
   pull_request:
 
 jobs:
@@ -23,3 +24,36 @@ jobs:
 
       - name: Test
         run: cargo test --workspace --verbose
+
+  build-windows-client:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'push'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust (stable) + Windows target
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          targets: x86_64-pc-windows-gnu
+
+      - name: Install MinGW cross-compiler
+        run: sudo apt-get update && sudo apt-get install -y gcc-mingw-w64-x86-64
+
+      - name: Cache dependencies
+        uses: Swatinem/rust-cache@v2
+        with:
+          key: windows-gnu
+
+      - name: Build Windows client (cross-compile)
+        run: |
+          cargo build --release --package helios-client --target x86_64-pc-windows-gnu
+        env:
+          CARGO_TARGET_X86_64_PC_WINDOWS_GNU_LINKER: x86_64-w64-mingw32-gcc
+
+      - name: Upload Windows client artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: helios-remote-client-windows
+          path: target/x86_64-pc-windows-gnu/release/helios-client.exe
+          if-no-files-found: error