Replace on-screen action buttons with right-click context menus

The tree and main panels now behave like a native file manager: right
click (or the keyboard Menu key) on a folder, asset, breadcrumb crumb, or
empty tree/panel background brings up the relevant actions instead of a
row of always-visible buttons.

- New generic showContextMenu() in app.js with per-target-kind menu
  builders (folder/asset/tree background/panel background), gated by
  admin role the same way the old buttons were.
- Removed: the "..." tree-row action button, and the New Folder/Add
  Asset/Rename/Delete/Edit/Move/Duplicate/Import/Export buttons from the
  inventory toolbar and every panel. Expand All/Collapse All shrank to two
  icon-only buttons next to the search box; everything else creation- or
  mutation-related is right-click only.
- Replaced the per-asset embedded "Move" modal with one shared
  #moveAssetModal populated via a new GET /app/folders/options endpoint,
  since move can now be triggered from any asset row, not just its own
  detail view.
- Breadcrumb crumbs (including the current folder) and folder/asset table
  rows are now valid right-click targets too, not just tree rows.

web/templates/pages/inventory.html

@@ -2,19 +2,13 @@
 <div class="inventory-shell d-flex">
   <aside id="tree-pane" class="tree-pane border-end d-flex flex-column">
     <div class="p-2 border-bottom">
-      <div class="input-group input-group-sm mb-2">
+      <div class="input-group input-group-sm">
         <span class="input-group-text"><i class="bi bi-search"></i></span>
         <input type="search" id="tree-search" class="form-control" placeholder="Search inventory&hellip;">
+        <button class="btn btn-outline-secondary" id="btn-expand-all" title="Expand all folders"><i class="bi bi-arrows-expand"></i></button>
+        <button class="btn btn-outline-secondary" id="btn-collapse-all" title="Collapse all folders"><i class="bi bi-arrows-collapse"></i></button>
       </div>
-      <div class="d-flex gap-1 flex-wrap">
-        <button class="btn btn-sm btn-outline-secondary" id="btn-expand-all" title="Expand all folders"><i class="bi bi-arrows-expand"></i> Expand</button>
-        <button class="btn btn-sm btn-outline-secondary" id="btn-collapse-all" title="Collapse all folders"><i class="bi bi-arrows-collapse"></i> Collapse</button>
-        {{if .User.IsAdmin}}
-        <button class="btn btn-sm btn-primary ms-auto" id="btn-new-root-folder" data-parent-id="" title="New top-level folder">
-          <i class="bi bi-folder-plus"></i>
-        </button>
-        {{end}}
-      </div>
+      <div class="form-text">Right-click the tree to add, rename, or delete.</div>
     </div>
     <div class="tree-scroll flex-fill overflow-auto">
       <ul class="tree list-unstyled mb-0" id="tree-root">