Restructure frontend into templates/ and static/

- Move index.html → templates/index.html (Jinja2, uses url_for)
- Extract CSS → static/css/app.css
- Split JS into static/js/: utils.js, dashboard.js, shares.js,
  quotas.js, users.js, main.js
- Switch app.py from send_file to render_template

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -5,7 +5,7 @@ import subprocess
 import functools
 from pathlib import Path
 
-from flask import Flask, request, jsonify, send_file, Response
+from flask import Flask, request, jsonify, render_template, Response
 
 app = Flask(__name__)
 
@@ -130,7 +130,7 @@ def add_nfs_share(path, clients, ro=False):
 @app.route('/')
 @require_auth
 def index():
-    return send_file(os.path.join(os.path.dirname(__file__), 'index.html'))
+    return render_template('index.html')
 
 
 @app.route('/api/shares', methods=['GET'])