feat(proxy): cross-platform desktop GUI (opt-in), CLI stays headless & cross-platform

Makefile

@@ -1,6 +1,6 @@
 SHELL := /bin/bash
 
-.PHONY: all engine engine-http3 engine-test proxy proxy-test platform-install python-check viz-test smoke docker clean help
+.PHONY: all engine engine-http3 engine-test proxy proxy-gui proxy-test platform-install python-check viz-test smoke docker clean help
 
 all: engine ## Build the execution engine (default)
 
@@ -20,6 +20,9 @@ engine-test: ## Run the engine's unit tests
 proxy: ## Build the intercepting proxy + MCP server -> proxy-rs/target/release/offat-proxy
 	cd proxy-rs && cargo build --release
 
+proxy-gui: ## Build the intercepting proxy with the cross-platform desktop GUI (offat-proxy gui)
+	cd proxy-rs && cargo build --release --features gui
+
 proxy-test: ## Run the proxy's unit tests
 	cd proxy-rs && cargo test
 

docs/proxy.md

@@ -91,6 +91,21 @@ count. Press `/` to filter history with the same expression language the rules u
 `?` shows help, `q` quits. It reads the flow store read-only and never alters
 traffic.
 
+## Desktop GUI (cross-platform, opt-in)
+
+```bash
+cargo build --release --features gui
+offat-proxy gui --scope api.example.test
+```
+
+A native window (egui/eframe) that runs on Linux, macOS and Windows, with the same
+live flow list, detail pane, scope/rule status and expression filter as the
+terminal UI, plus mouse selection. The GUI is an **opt-in build feature** so the
+default binary stays a lean, cross-platform CLI with no windowing dependencies —
+the CLI, the terminal UI (`tui`) and MCP all work everywhere without it. A build
+without the feature still recognises the `gui` command and tells you how to enable
+it.
+
 ## Rules and filters (live block / allow / tag / match-replace)
 
 A small, safe expression interpreter ("bambdas") matches flows and acts on them