fix(plugins): move the repository URL to the gear menu and drop the no-repository hint (#1282)

The Available tab always showed a "There is no online plugin repository"
notice under the search field when no index was set. Release builds use
the official index by default, so the notice only appeared in development
builds, where the empty URL field above it already said the same thing.
The URL field itself sat at the top of the list although most people never
change it.

- The notice is gone, and the search placeholder reads "Search plugins"
  because the list holds marketplace plugins as well as local ones.
- With a repository configured, the empty details panel says "No plugin
  selected" instead of claiming there is no online repository.
- The marketplace index URL is edited from the gear menu's "Plugin
  Repository…" dialog; leaving it empty restores the default.

src/features/plugins/PluginsPage.module.css

@@ -369,17 +369,6 @@
   padding: 4px 8px;
 }
 
-.marketBar {
-  display: flex;
-  gap: 4px;
-  align-items: center;
-}
-
-.marketBar > :first-child {
-  flex: 1;
-  min-width: 0;
-}
-
 .categorySelect {
   width: 100%;
 }