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>

docs/installation.md

@@ -3,7 +3,16 @@
 Dash ships as a single container image or a single static binary. All state
 lives in one directory: a SQLite database and uploaded icons.
 
-## Docker (recommended)
+## One-line install
+
+```sh
+curl -fsSL https://raw.githubusercontent.com/Dvorinka/Dash/main/install.sh | sh
+```
+
+Creates `./dash`, pulls the image, and starts on port 3000. Override with
+`DASH_DIR` / `DASH_PORT` environment variables.
+
+## Docker (manual)
 
 ```sh
 docker run -d --name dash \