feat: add proxy support for SDK downloads (v1.1.0)

Add --proxy and --no-proxy global flags to control HTTP/HTTPS proxy
usage for all network operations (SDK installs, FTC SDK clone/fetch,
Android SDK download).

Proxy resolution priority:
  1. --no-proxy          → go direct, ignore everything
  2. --proxy <url>       → use the specified proxy
  3. HTTPS_PROXY / HTTP_PROXY env vars (auto-detected)
  4. Nothing             → go direct

Key implementation details:
- reqwest client is always built through ProxyConfig::client() rather
  than Client::new(), so --no-proxy actively suppresses env-var
  auto-detection instead of just being a no-op.
- git2/libgit2 has its own HTTP transport that doesn't use reqwest.
  GitProxyGuard is an RAII guard that temporarily sets/clears the
  HTTPS_PROXY env vars around clone and fetch operations, then restores
  the previous state on drop. This avoids mutating ~/.gitconfig.
- Gradle wrapper reads HTTPS_PROXY natively; no programmatic
  intervention needed.
- All network failure paths now print offline/air-gapped installation
  instructions automatically, covering manual SDK installs and Gradle
  distribution download.

Closes: v1.1.0 proxy support milestone

src/sdk/mod.rs

@@ -6,6 +6,7 @@ use colored::*;
 pub mod android;
 pub mod ftc;
 pub mod gradle;
+pub mod proxy;
 
 pub struct SdkConfig {
     pub ftc_sdk_path: PathBuf,