Initial commit: Triple-C app, container, and CI

Tauri v2 desktop app (React/TypeScript + Rust) for managing
containerized Claude Code environments. Includes Gitea Actions
workflow for building and pushing the sandbox container image,
and a BUILDING.md guide for manual app builds on Linux and Windows.

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

app/src-tauri/src/docker/client.rs

@@ -0,0 +1,23 @@
+use bollard::Docker;
+use std::sync::OnceLock;
+
+static DOCKER: OnceLock<Result<Docker, String>> = OnceLock::new();
+
+pub fn get_docker() -> Result<&'static Docker, String> {
+    let result = DOCKER.get_or_init(|| {
+        Docker::connect_with_local_defaults()
+            .map_err(|e| format!("Failed to connect to Docker daemon: {}", e))
+    });
+    match result {
+        Ok(docker) => Ok(docker),
+        Err(e) => Err(e.clone()),
+    }
+}
+
+pub async fn check_docker_available() -> Result<bool, String> {
+    let docker = get_docker()?;
+    match docker.ping().await {
+        Ok(_) => Ok(true),
+        Err(e) => Err(format!("Docker daemon not responding: {}", e)),
+    }
+}