feat(ui): drag-drop capture onto the main panel

Drops into #main POST /api/capture/upload one file at a time, with
space_id pre-filled from localStorage.last_space_id (set whenever the
space view renders).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

public/views/space.js

@@ -33,6 +33,7 @@ export async function render(main, ctx) {
     el('p', { class: 'view-sub muted' }, 'Loading …')
   );
 
+  localStorage.setItem('last_space_id', id);
   let space;
   try { space = await api.get('/api/spaces/' + id); }
   catch (e) {