feat: full inline issue editing (title + body + assignee)

Replaced bare prompt() with proper edit form in card modal:
- showEditForm: renders title input + body textarea + assignee field
- saveIssueEdit: PATCH /api/issues/... with title, body, assignee
- 'Annuler' button returns to card detail view
- Old editIssueInline (prompt only) removed
- Added .form-label CSS style

static/css/app.css

@@ -199,6 +199,7 @@ body { min-height: 100vh; }
   margin-bottom: 8px;
 }
 .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; }
 .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; }