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

crates/client/Cargo.toml

@@ -3,8 +3,27 @@ name = "helios-client"
 version = "0.1.0"
 edition = "2021"
 
-# Phase 2 — Windows client (not yet implemented)
-# See README.md in this crate for the planned implementation.
+[[bin]]
+name = "helios-client"
+path = "src/main.rs"
 
 [dependencies]
+tokio = { version = "1", features = ["full"] }
+tokio-tungstenite = { version = "0.21", features = ["connect"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
 helios-common = { path = "../common" }
+dirs = "5"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+base64 = "0.22"
+png = "0.17"
+futures-util = "0.3"
+
+[target.'cfg(windows)'.dependencies]
+windows = { version = "0.54", features = [
+    "Win32_Foundation",
+    "Win32_Graphics_Gdi",
+    "Win32_UI_Input_KeyboardAndMouse",
+    "Win32_System_Threading",
+] }