feat: multi-label selection + double-click column to create issue

- Labels now rendered as checkboxes (not single-select dropdown)
  in both create modal and edit form
- ALL_LABELS and ALL_MILESTONES passed as JSON to JS
- Double-click on empty column area opens create modal
  (pre-fills with that column name in title placeholder context)
- '+ Issue' button also opens the same modal
- Removed old inline new-issue-form (Alpine x-show), replaced with modal
- Create modal: title, body, multi-label checkboxes, milestone, assignee
- Edit form: title, body, multi-label checkboxes, assignee
- CSS: .label-checkboxes + .label-cb styles

static/css/app.css

@@ -200,6 +200,9 @@ body { min-height: 100vh; }
 }
 .form-input:focus, .form-select:focus { outline: none; border-color: var(--accent); }
 .form-label { font-size: 12px; color: var(--text-dim); display: block; margin: 8px 0 4px 0; }
+.label-checkboxes { display: flex; flex-wrap: wrap; gap: 4px; margin-bottom: 4px; }
+.label-cb { display: flex; align-items: center; gap: 3px; font-size: 12px; cursor: pointer; }
+.label-cb input[type="checkbox"] { cursor: pointer; }
 .form-input-sm { background: var(--bg); border: 1px solid var(--border); border-radius: 4px; padding: 4px 6px; color: var(--text); font-size: 12px; }
 .form-select-sm { background: var(--bg); border: 1px solid var(--border); border-radius: 4px; padding: 2px 4px; color: var(--text); font-size: 12px; }
 .form-row { display: flex; gap: 8px; }