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/10-layout.css

@@ -1,5 +1,6 @@
 .app-shell {
   width: min(86rem, calc(100% - 2rem));
+  max-width: 100%;
   margin: 0 auto;
   padding: 2rem 0;
   display: grid;
@@ -8,6 +9,7 @@
 }
 
 .app-sidebar {
+  min-width: 0;
   position: sticky;
   top: 5rem;
   align-self: start;
@@ -20,6 +22,7 @@
 }
 
 .sidebar-link {
+  min-width: 0;
   display: flex;
   align-items: center;
   gap: 0.55rem;
@@ -30,6 +33,13 @@
   text-decoration: none;
 }
 
+.sidebar-link span {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 .sidebar-link:hover,
 .sidebar-link.is-active {
   border-color: var(--border);
@@ -100,7 +110,7 @@
 
 .inline-controls input,
 .inline-controls select {
-  min-width: 15rem;
+  min-width: min(15rem, 100%);
 }
 
 .compact-input {
@@ -108,10 +118,18 @@
 }
 
 .settings-form {
+  min-width: 0;
   display: grid;
   gap: 1.5rem;
 }
 
+.settings-form > *,
+.settings-section > *,
+.tabs-bar > *,
+.tab-list > * {
+  min-width: 0;
+}
+
 .settings-form-narrow {
   grid-template-columns: minmax(0, 1fr);
   gap: 0.9rem;
@@ -207,6 +225,7 @@
   top: calc(100% + 0.5rem);
   z-index: 10;
   width: 15rem;
+  max-width: min(15rem, calc(100vw - 2rem));
   padding: 1rem;
   background: color-mix(in srgb, var(--card) 97%, #000);
   border: 1px solid var(--border);
@@ -226,6 +245,7 @@
 /* Copyable URL field */
 .copy-field {
   display: flex;
+  min-width: 0;
   gap: 0.5rem;
   align-items: center;
   margin-top: 0.75rem;