feat: add cross-platform support via platform detection module

Introduces src/platform.rs with OS detection and env var overrides.
All hardcoded Linux paths replaced with Platform::detect() across
8 source files. Key changes:

- New Platform struct with 11 fields (all overridable via env vars)
- /proc/ access gated to Linux (#[cfg(target_os = "linux")])
- pgrep/pkill patterns broadened for cross-platform LS discovery
- sec-ch-ua-platform header now dynamic per OS
- Token, traces, config, CA cert paths use platform module
- LD_PRELOAD DNS redirect gated to Linux only
- Setup scripts for Linux (systemd) and macOS (launchd)
- find_ls_binary_path has cross-platform stubs

All 46 tests pass, cargo check clean.

scripts/setup-linux.sh

@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+# ZeroGravity — Linux setup
+# Creates the zerogravity-ls system user for UID-scoped iptables isolation,
+# installs the systemd user service, and builds the dns_redirect.so preload lib.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
+
+# ── 1. System user for UID isolation ──
+echo "→ Creating zerogravity-ls system user…"
+if id -u zerogravity-ls &>/dev/null; then
+    echo "  Already exists."
+else
+    sudo useradd --system --no-create-home --shell /usr/sbin/nologin zerogravity-ls
+    echo "  Created."
+fi
+
+# ── 2. Sudoers rule (run commands as zerogravity-ls without password) ──
+SUDOERS="/etc/sudoers.d/zerogravity"
+echo "→ Installing sudoers rule…"
+if [ -f "$SUDOERS" ]; then
+    echo "  Already exists."
+else
+    echo "$USER ALL=(zerogravity-ls) NOPASSWD: ALL" | sudo tee "$SUDOERS" > /dev/null
+    sudo chmod 0440 "$SUDOERS"
+    echo "  Installed: $SUDOERS"
+fi
+
+# ── 3. Data directory permissions ──
+echo "→ Setting up /tmp/zerogravity-standalone…"
+sudo mkdir -p /tmp/zerogravity-standalone
+sudo chmod 1777 /tmp/zerogravity-standalone
+
+# ── 4. Config directory ──
+echo "→ Setting up ~/.config/zerogravity…"
+mkdir -p "$HOME/.config/zerogravity"
+
+# ── 5. Systemd user service ──
+echo "→ Installing systemd user service…"
+UNIT_DIR="$HOME/.config/systemd/user"
+mkdir -p "$UNIT_DIR"
+cat > "$UNIT_DIR/zerogravity.service" << EOF
+[Unit]
+Description=ZeroGravity Proxy
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=$PROJECT_DIR/target/release/zerogravity
+WorkingDirectory=$PROJECT_DIR
+Environment=RUST_LOG=info
+Restart=on-failure
+RestartSec=3
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=default.target
+EOF
+systemctl --user daemon-reload
+echo "  Installed: $UNIT_DIR/zerogravity.service"
+echo "  Enable with: systemctl --user enable zerogravity"
+
+# ── 6. Build ──
+echo "→ Building release binary…"
+cd "$PROJECT_DIR"
+cargo build --release 2>&1 | tail -1
+echo ""
+echo "✓ Setup complete. Start with: zg start"