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>

static/js/utils.js

@@ -0,0 +1,28 @@
+function esc(s) {
+    return String(s ?? '')
+        .replace(/&/g, '&amp;').replace(/</g, '&lt;')
+        .replace(/>/g, '&gt;').replace(/"/g, '&quot;');
+}
+
+function showToast(msg, type = 'success') {
+    const colors = { success: '#3fb950', danger: '#f85149', warning: '#d29922' };
+    const id = 'toast-' + Date.now();
+    document.getElementById('toastContainer').insertAdjacentHTML('beforeend', `
+        <div id="${id}" class="toast align-items-center border-0 mb-2" role="alert"
+             style="background:#161b22; border-left:3px solid ${colors[type] ?? colors.success} !important; color:#e6edf3;">
+            <div class="d-flex">
+                <div class="toast-body">${esc(msg)}</div>
+                <button type="button" class="btn-close btn-close-white me-2 m-auto" data-bs-dismiss="toast"></button>
+            </div>
+        </div>`);
+    const el = document.getElementById(id);
+    const t = new bootstrap.Toast(el, { delay: 3500 });
+    t.show();
+    el.addEventListener('hidden.bs.toast', () => el.remove());
+}
+
+async function apiFetch(url, opts) {
+    const res = await fetch(url, opts);
+    if (res.status === 401) { location.reload(); return null; }
+    return res;
+}