fix(FN-2492): polish plugin manager bundled runtime UX

- Add a Bundled Runtime Plugins section with one-click install actions, install-state badges, and empty-state discoverability improvements
- Refine plugin detail semantics and controls by tightening heading hierarchy, button variants, and mobile/toggle styling behavior
- Expand PluginManager test coverage for bundled runtime installation, installed-state disabling, and heading-level accessibility contracts
- Clean up dashboard CSS ownership by removing duplicated global/toggle rules, updating surface-hover usage, and documenting bundled runtime discovery in settings docs

packages/dashboard/app/styles.css

@@ -1100,51 +1100,6 @@ input[type="range"]:focus-visible {
   margin-top: var(--space-xl);
 }
 
-.settings-plugins-subsection-toggle {
-  display: inline-flex;
-  gap: var(--space-xs);
-  padding: 0 var(--space-xl);
-  margin: var(--space-md) 0;
-}
-
-.settings-plugins-subsection-btn {
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  border: var(--btn-border-width) solid var(--border);
-  border-radius: var(--radius-pill);
-  background: var(--surface);
-  color: var(--text-muted);
-  font-size: 0.85rem;
-  font-weight: 600;
-  padding: var(--space-xs) var(--space-md);
-  cursor: pointer;
-  transition:
-    background var(--transition-fast),
-    color var(--transition-fast),
-    border-color var(--transition-fast),
-    box-shadow var(--transition-fast);
-}
-
-.settings-plugins-subsection-btn:hover {
-  background: var(--surface-hover);
-  color: var(--text);
-}
-
-.settings-plugins-subsection-btn:focus-visible {
-  outline: none;
-  box-shadow: var(--focus-ring-strong);
-}
-
-.settings-plugins-subsection-btn.active {
-  background: color-mix(in srgb, var(--todo) 14%, transparent);
-  border-color: color-mix(in srgb, var(--todo) 45%, var(--border));
-  color: var(--todo);
-}
-
-.settings-plugins-subsection-panel {
-  padding-bottom: var(--space-md);
-}
 
 /* Scope indicators in sidebar nav items */
 .settings-scope-icon {
@@ -2950,17 +2905,6 @@ input[type="range"]:focus-visible {
     margin: 0 var(--space-lg) 0;
   }
 
-  .settings-plugins-subsection-toggle {
-    display: flex;
-    width: 100%;
-    padding: 0 var(--space-lg);
-  }
-
-  .settings-plugins-subsection-btn {
-    flex: 1;
-    min-height: 36px;
-  }
-
   .form-group {
     padding: 0 14px;
   }