v0.4.0: add cross-platform desktop UI (egui)

New bin 'mhrv-rs-ui' behind the 'ui' feature flag. CLI users pay
zero egui compile cost; UI users get a single static binary.

UI features:
- Config form (Apps Script ID, auth key, Google IP, front domain,
  ports, log level, verify_ssl)
- Start/Stop buttons that spawn the proxy on a dedicated tokio thread
- Live stats (relay calls, failures, cache hit rate, bytes relayed,
  blacklisted scripts) polled every ~700ms
- Test button (end-to-end relay probe)
- Install CA / Check CA buttons
- Recent log panel (last 200 lines)
- Dense, dark, utility-look: no emojis, no cards, no gradients

Architecture:
- Refactored crate into lib + two bins (mhrv-rs, mhrv-rs-ui).
  src/lib.rs exposes all modules, main.rs uses them via 'use mhrv_rs::...'
- New src/data_dir.rs: platform-appropriate user data dir
  (~/Library/Application Support/mhrv-rs on macOS,
   ~/.config/mhrv-rs on Linux, %APPDATA%\mhrv-rs on Windows).
  CLI falls back to ./config.json for backward compat.
- CA moves to {data_dir}/ca/ca.crt (was ./ca/ca.crt).
- UI background thread owns the tokio runtime and proxy handle;
  communicates with UI via std::mpsc commands + Arc<Mutex<UiState>>.
- macOS .app bundle: assets/macos/Info.plist template + build-app.sh
  that assembles .app from the binary. Bundled into release zips.
- CI: Linux system libs (libxkbcommon, libwayland, libxcb*, libx11,
  libgl, libgtk-3) installed on Ubuntu runners for eframe. aarch64
  Linux UI is best-effort cross-compile. Windows MinGW, macOS native.

25 lib tests still pass. 5MB release UI binary on macOS.

assets/macos/Info.plist

@@ -0,0 +1,30 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>CFBundleName</key>
+    <string>mhrv-rs</string>
+    <key>CFBundleDisplayName</key>
+    <string>mhrv-rs</string>
+    <key>CFBundleIdentifier</key>
+    <string>com.therealaleph.mhrv-rs</string>
+    <key>CFBundleVersion</key>
+    <string>__VERSION__</string>
+    <key>CFBundleShortVersionString</key>
+    <string>__VERSION__</string>
+    <key>CFBundlePackageType</key>
+    <string>APPL</string>
+    <key>CFBundleExecutable</key>
+    <string>mhrv-rs-ui</string>
+    <key>CFBundleSignature</key>
+    <string>????</string>
+    <key>LSMinimumSystemVersion</key>
+    <string>10.14</string>
+    <key>NSHighResolutionCapable</key>
+    <true/>
+    <key>NSPrincipalClass</key>
+    <string>NSApplication</string>
+    <key>LSApplicationCategoryType</key>
+    <string>public.app-category.utilities</string>
+</dict>
+</plist>