Harden Linux server runtime and packaging

Adds end-to-end Linux compatibility work for the dedicated server: new CI workflow (Ubuntu + Arch), line-ending/executable safeguards, and a local venv-first startup flow with split dependency files for server vs optional host GUI tooling. Improves runtime resilience with better bind error messages, stricter config validation, writable-state checks, cleaner socket/thread shutdown behavior, and SIGTERM-aware graceful stop handling for headless/systemd use. Updates deployment/startup docs and adds portability/runtime integration tests to lock in these behaviors.

server/scripts/linux_compat_checks.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+ROOT="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)"
+cd -- "${ROOT}"
+
+echo "==> Checking start.sh line endings (LF)"
+python3 - <<'PY'
+from pathlib import Path
+data = Path("start.sh").read_bytes()
+if b"\r\n" in data or data.count(b"\r"):
+    raise SystemExit("ERROR: start.sh contains CR/CRLF line endings")
+PY
+
+echo "==> Checking start.sh is executable"
+test -x start.sh
+
+echo "==> bash -n start.sh"
+bash -n start.sh
+
+if command -v shellcheck >/dev/null 2>&1; then
+  echo "==> shellcheck start.sh"
+  shellcheck start.sh
+else
+  echo "WARNING: shellcheck not installed; skipping"
+fi
+
+echo "==> Creating clean virtual environment"
+rm -rf .venv-ci
+python3 -m venv .venv-ci
+.venv-ci/bin/python -m pip install --upgrade pip
+.venv-ci/bin/python -m pip install -r requirements-server.txt pytest
+
+echo "==> compileall"
+.venv-ci/bin/python -m compileall -q .
+
+echo "==> pytest"
+.venv-ci/bin/python -m pytest -q
+
+echo "All Linux compatibility checks passed."