Show a dashed drop-area in empty kanban columns and the inbox

.kanban__cards:empty (shared by every status column and the sidebar's
inbox list) gets a dashed border, transparent background, and a
'Drop cards here' hint, echoing the dashboard's 'Create a project'
tile. Pure CSS: both draggables are already always-rendered even when
empty, so an empty one is a genuinely childless element and :empty
just tracks that -- it steps aside on its own once Sortable inserts a
drag-over ghost, and needs no JS empty-state flag.

Drops the sidebar's separate 'Nothing in the inbox.' message, now
redundant with the box itself. Verified the container really is
childless when empty via a DOM dump against a running instance (no
screenshot).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/components/AppSidebar.vue

@@ -139,8 +139,9 @@ async function onInboxChange(change: ColumnChange) {
 
     <template v-else>
       <!-- Always rendered (even empty) so it stays a valid drop target for a
-           card dragged out of a project's kanban board. -->
-      <p v-if="inbox.cards.length === 0" class="sidebar__note muted">Nothing in the inbox.</p>
+           card dragged out of a project's kanban board -- empty shows a
+           dashed drop-area box (.kanban__cards:empty in style.css) instead
+           of a separate message. -->
       <draggable
         :list="inbox.cards"
         :group="{ name: 'kanban' }"