docs: OSS readiness — README hero, one-line installer, stale doc fixes

Restructure README to the standard OSS layout (logo, badges, screenshots
grid, architecture, quick start). Add install.sh for curl|sh installs with
a named-volume fallback for Docker Desktop file-sharing limits, and
scripts/setup.sh for clone-based setup. Correct SECURITY.md (no longer
pre-1.0) and CONTRIBUTING.md (auth shipped in 2.0.0).

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

scripts/setup.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Dash setup from a clone — builds the image locally and starts the stack.
+# For the prebuilt image path, use ../install.sh instead.
+set -eu
+
+cd "$(dirname "$0")/.."
+
+command -v docker >/dev/null 2>&1 || { echo "error: docker not found" >&2; exit 1; }
+docker compose version >/dev/null 2>&1 || { echo "error: docker compose plugin not found" >&2; exit 1; }
+
+docker compose up -d --build
+
+echo ""
+echo "Dash is up: http://localhost:3000"
+echo "Logs: docker compose logs -f"
+echo ""
+echo "For development instead:"
+echo "  npm install && npm run dev          # vite on :5173"
+echo "  cd apps/backend && DASH_DEV=1 go run ./cmd/dash   # API on :8080"