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/partials/panel_root.html

@@ -1,8 +1,8 @@
 {{define "panel_root"}}
-<div class="panel-content p-3">
+<div class="panel-content p-3" data-panel-type="root">
   <nav aria-label="breadcrumb">
     <ol class="breadcrumb mb-2">
-      <li class="breadcrumb-item active" aria-current="page">Inventory</li>
+      <li class="breadcrumb-item active" aria-current="page" data-type="root">Inventory</li>
     </ol>
   </nav>
 
@@ -62,14 +62,7 @@
     </div>
   </div>
 
-  <div class="d-flex align-items-center mb-2">
-    <h6 class="mb-0">Top-Level Folders</h6>
-    <div class="ms-auto d-flex gap-2">
-      <a class="btn btn-sm btn-outline-secondary" href="/export/assets.csv"><i class="bi bi-download"></i> Export CSV</a>
-      <button class="btn btn-sm btn-outline-secondary admin-only" data-bs-toggle="modal" data-bs-target="#importModal"><i class="bi bi-upload"></i> Import CSV</button>
-      <button class="btn btn-sm btn-primary admin-only" id="btn-new-folder-root"><i class="bi bi-folder-plus"></i> New Folder</button>
-    </div>
-  </div>
+  <h6 class="mb-2">Top-Level Folders</h6>
 
   {{if .Folders}}
   <div class="list-group mb-3">
@@ -83,7 +76,7 @@
     {{end}}
   </div>
   {{else}}
-  <p class="text-body-secondary">No folders yet. {{if true}}<span class="admin-only">Create one to start organizing your inventory.</span>{{end}}</p>
+  <p class="text-body-secondary">No folders yet. <span class="admin-only">Right-click here (or the tree) to create one.</span></p>
   {{end}}
 
   {{if .UnfiledAssets}}