ref(frontend): Replace Bootstrap with Tailwind CSS

Swap the Bootstrap CDN for a self-hosted Tailwind CSS build, mirroring
the myice setup: standalone Tailwind CLI v3.4.17, a one-off build-css.sh
script, and a gitignored generated style.css.

- Add tailwind.config.js, style-input.css, build-css.sh at repo root
- Rewrite templates/index.html: drop Bootstrap link/script, link
  /style.css, convert all classes to Tailwind utilities; showAlert now
  uses a class lookup so the JIT scanner detects dynamic classes
- Add /style.css FileResponse route in main.py; drop cdn.jsdelivr.net
  from script-src/style-src in the CSP
- Flip test assertion to assert cdn.jsdelivr.net is absent from CSP

The generated style.css and tailwindcss binary are gitignored; run
./build-css.sh to regenerate (or --watch for dev).

build-css.sh

@@ -0,0 +1,41 @@
+#!/bin/bash
+set -euo pipefail
+
+# Build CSS using Tailwind CSS standalone CLI
+# For local development, run:
+#   ./build-css.sh        # one-off build
+#   ./build-css.sh --watch  # watch mode
+
+version="v3.4.17"
+os="$(uname -s)"
+arch="$(uname -m)"
+
+case "$os" in
+    Linux*)     platform="linux";;
+    Darwin*)    platform="macos";;
+    *)          echo "Unsupported OS: $os"; exit 1;;
+esac
+
+case "$arch" in
+    x86_64)     arch="x64";;
+    arm64)      arch="arm64";;
+    aarch64)    arch="arm64";;
+    *)          echo "Unsupported architecture: $arch"; exit 1;;
+esac
+
+binary="tailwindcss-${platform}-${arch}"
+url="https://github.com/tailwindlabs/tailwindcss/releases/download/${version}/${binary}"
+
+if [ ! -f "tailwindcss" ]; then
+    echo "Downloading Tailwind CSS standalone CLI (${version} ${platform}/${arch})..."
+    curl -sL "$url" -o tailwindcss
+    chmod +x tailwindcss
+fi
+
+if [ "${1:-}" == "--watch" ]; then
+    echo "Building CSS in watch mode..."
+    ./tailwindcss -i ./style-input.css -o ./style.css --watch
+else
+    echo "Building CSS..."
+    ./tailwindcss -i ./style-input.css -o ./style.css --minify
+fi