feat(ru): full Russia/offline support (#RF)

- Add lib/download_helper.sh: proxy detection, GitHub mirrors, offline cache, tarball fallback
- Rewrite install_nfqws.sh: offline bundle support, bol-van/zapret via git clone + tarball + mirror fallback
- Rewrite sync_from_upstream.sh: proxy aware, --offline flag, graceful degradation
- Add prepare_offline_bundle.sh: create tar.gz with sources + lists + fake bins on machine WITH internet
- Add .env.example: GITHUB_PROXY, HTTPS_PROXY configuration
- Update install.sh: check GitHub reachability, suggest proxy/offline, --offline flag
- Update README: Russia section at top, Offline section, VPN/proxy examples, .env guide

.env.example

@@ -0,0 +1,10 @@
+# Zapret for Linux — Environment Configuration
+# Copy this to .env and set your proxy/mirror
+
+# HTTP/HTTPS proxy for GitHub access in Russia
+# GITHUB_PROXY=http://proxy.local:8080
+# GITHUB_PROXY=socks5://127.0.0.1:1080
+
+# Alternative: set system-wide proxy
+# HTTP_PROXY=http://proxy.local:8080
+# HTTPS_PROXY=http://proxy.local:8080