feat(dev): add portable development launcher (#50)

Validate the local runtime and dependencies before starting the development server from the repository root.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

start.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
+cd -- "$repo_dir"
+
+if ! command -v node >/dev/null 2>&1; then
+    printf '%s\n' 'Error: Node.js >=18 is required. Install Node.js and try again.' >&2
+    exit 1
+fi
+
+if ! node -e 'process.exit(Number(process.versions.node.split(".")[0]) >= 18 ? 0 : 1)'; then
+    printf '%s\n' 'Error: Node.js >=18 is required. Upgrade Node.js and try again.' >&2
+    exit 1
+fi
+
+if ! command -v npm >/dev/null 2>&1; then
+    printf '%s\n' 'Error: npm is required. Install npm and try again.' >&2
+    exit 1
+fi
+
+if [[ ! -d node_modules ]] || ! npm ls --depth=0 --omit=optional >/dev/null 2>&1; then
+    printf '%s\n' 'Error: Dependencies are missing or invalid. Run:' >&2
+    printf '  cd %q && npm ci\n' "$repo_dir" >&2
+    exit 1
+fi
+
+printf '%s\n' 'Starting Nodecast TV: http://localhost:3000'
+exec npm run dev