feat(collections): browse a collection from the sidebar

The sidebar gains the concept's COLLECTIONS group (D136): one row per
collection with its folder icon, name and listed-member count, without the
PROPOSED badge; the label shows once a collection exists. A row opens
#/collection/<id>, the Images page filtered to that collection (D137): the
collection's name as h1, Collection as breadcrumb and title, Edit to rename
or delete it through the saved views' Edit dialog with collection nouns, and
a note counting members the sources do not list now. An unknown or deleted
collection shows a page that says so with a link to Images. A saved view on
a deleted collection lists nothing and says so (MC6, D140). The list is read
again after a restore and the counts 300 ms after a change.

Part of #331

web/workspace.html

@@ -25,6 +25,9 @@
             <!-- D49: the concept's SAVED VIEWS group without its "PROPOSED" badge (saved views are persisted, #249 D16). -->
             <div class="sidebar-label">SAVED VIEWS</div>
             <nav id="saved-nav" aria-label="Saved views"></nav>
+            <!-- D136: the concept's COLLECTIONS group without its "PROPOSED" badge; the label shows once a collection is listed. -->
+            <div class="sidebar-label" hidden>COLLECTIONS</div>
+            <nav id="collections-nav" aria-label="Collections"></nav>
             <div class="sidebar-bottom">
                 <nav id="system-nav" aria-label="Tools and system"></nav>
                 <div class="connection"><i></i><div>ComfyUI workspace<small>Your library, on your server</small></div></div>