feat(storage): support deleting backends and improve admin UI

- Implement storage backend deletion, which automatically resets default storage settings and user-specific overrides when a backend is removed.
- Add unit tests covering the delete action and its cleanup side effects.
- Improve admin UI responsiveness, fixing table scrolling, flex wrapping, and text truncation for long storage backend names.
- Update security documentation to clarify trusted proxy configurations and explain how trusted proxies are protected from automatic bans.

backend/static/css/17-gruvbox.css

@@ -0,0 +1,88 @@
+/*
+ * Gruvbox theme polish.
+ *
+ * Core colour tokens live in 00-base.css. This file adds the warmer, grounded
+ * Gruvbox-specific surface treatment without changing layout behavior.
+ */
+
+:root[data-theme="gruvbox"] .site-header {
+  border-bottom-color: rgba(250, 189, 47, 0.2);
+  backdrop-filter: blur(16px) saturate(130%);
+  -webkit-backdrop-filter: blur(16px) saturate(130%);
+}
+
+:root[data-theme="gruvbox"] .brand-mark {
+  background: linear-gradient(135deg, #d79921, #fe8019);
+  color: #1d2021;
+  box-shadow: 0 8px 22px rgba(254, 128, 25, 0.22);
+}
+
+:root[data-theme="gruvbox"] .card,
+:root[data-theme="gruvbox"] .app-sidebar,
+:root[data-theme="gruvbox"] .storage-card,
+:root[data-theme="gruvbox"] .storage-op-card,
+:root[data-theme="gruvbox"] .metric-card,
+:root[data-theme="gruvbox"] .logs-filter-card {
+  background: color-mix(in srgb, var(--card) 92%, #1d2021);
+  border-color: rgba(235, 219, 178, 0.16);
+}
+
+:root[data-theme="gruvbox"] .admin-shell .app-sidebar {
+  border-color: rgba(250, 189, 47, 0.32);
+  background: linear-gradient(180deg, rgba(215, 153, 33, 0.12), rgba(40, 40, 40, 0.94));
+}
+
+:root[data-theme="gruvbox"] .admin-shell .sidebar-link.is-active {
+  border-color: rgba(250, 189, 47, 0.36);
+  background: rgba(215, 153, 33, 0.14);
+}
+
+:root[data-theme="gruvbox"] .admin-shell .kicker,
+:root[data-theme="gruvbox"] .kicker {
+  color: #fabd2f;
+}
+
+:root[data-theme="gruvbox"] h1 {
+  color: #fbf1c7;
+}
+
+:root[data-theme="gruvbox"] .button-primary,
+:root[data-theme="gruvbox"] .button.is-active {
+  border-color: rgba(250, 189, 47, 0.3);
+  background: linear-gradient(135deg, #d79921, #fabd2f);
+  color: #1d2021;
+  box-shadow: 0 10px 24px rgba(215, 153, 33, 0.2);
+}
+
+:root[data-theme="gruvbox"] .button-primary:hover {
+  background: linear-gradient(135deg, #fabd2f, #fe8019);
+}
+
+:root[data-theme="gruvbox"] .button-outline,
+:root[data-theme="gruvbox"] .button-ghost:hover,
+:root[data-theme="gruvbox"] .button-outline:hover {
+  border-color: rgba(235, 219, 178, 0.2);
+}
+
+:root[data-theme="gruvbox"] .badge-active,
+:root[data-theme="gruvbox"] .storage-detail-test.is-ok > span:last-child {
+  color: #b8bb26;
+}
+
+:root[data-theme="gruvbox"] .badge-disabled,
+:root[data-theme="gruvbox"] .storage-detail-test.is-err > span:last-child,
+:root[data-theme="gruvbox"] .form-error {
+  color: #fb4934;
+}
+
+:root[data-theme="gruvbox"] input:focus,
+:root[data-theme="gruvbox"] select:focus,
+:root[data-theme="gruvbox"] textarea:focus {
+  border-color: #fe8019;
+  box-shadow: 0 0 0 3px rgba(254, 128, 25, 0.18);
+}
+
+:root[data-theme="gruvbox"] ::selection {
+  background: #d79921;
+  color: #1d2021;
+}