Add dark theme and secret key creation script

scripts/generate-secret-key.ps1

@@ -0,0 +1,9 @@
+# Prints a cryptographically random SECRET_KEY value; does not modify .env.
+$bytes = [byte[]]::new(32)
+$rng = [System.Security.Cryptography.RNGCryptoServiceProvider]::new()
+try {
+    $rng.GetBytes($bytes)
+} finally {
+    $rng.Dispose()
+}
+($bytes | ForEach-Object { $_.ToString("x2") }) -join ""

scripts/generate-secret-key.sh

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+#!/bin/sh
+set -eu
+
+# Prints a cryptographically random SECRET_KEY value; does not modify .env.
+if command -v python3 >/dev/null 2>&1; then
+    python3 -c 'import secrets; print(secrets.token_hex(32))'
+elif command -v openssl >/dev/null 2>&1; then
+    openssl rand -hex 32
+else
+    printf '%s\n' "Neither python3 nor openssl is available to generate a secret." >&2
+    exit 1
+fi