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/android.rs

@@ -1,16 +1,17 @@
 use std::path::Path;
 use anyhow::{Result, Context};
 use indicatif::{ProgressBar, ProgressStyle};
-use reqwest::blocking::Client;
 use std::fs::File;
 use std::io::Write;
 use colored::*;
 
+use super::proxy::ProxyConfig;
+
 const ANDROID_SDK_URL_LINUX: &str = "https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip";
 const ANDROID_SDK_URL_MAC: &str = "https://dl.google.com/android/repository/commandlinetools-mac-11076708_latest.zip";
 const ANDROID_SDK_URL_WINDOWS: &str = "https://dl.google.com/android/repository/commandlinetools-win-11076708_latest.zip";
 
-pub fn install(sdk_path: &Path) -> Result<()> {
+pub fn install(sdk_path: &Path, proxy: &ProxyConfig) -> Result<()> {
     // Check if SDK exists AND is complete
     if sdk_path.exists() {
         match verify(sdk_path) {
@@ -42,10 +43,14 @@ pub fn install(sdk_path: &Path) -> Result<()> {
     
     // Download
     println!("Downloading from: {}", url);
-    let client = Client::new();
+    proxy.print_status();
+    let client = proxy.client()?;
     let response = client.get(url)
         .send()
-        .context("Failed to download Android SDK")?;
+        .map_err(|e| {
+            super::proxy::print_offline_instructions();
+            anyhow::anyhow!("Failed to download Android SDK: {}", e)
+        })?;
     
     let total_size = response.content_length().unwrap_or(0);