Fix ticket age, bulk assign, add column visibility toggle

- TicketView: ticket age was measuring from last update not creation;
  fixed to always use created_at
- dashboard.js: bulk assign used non-existent onSelect callback (no
  selection was ever stored); fixed to onChange with selected[0],
  added max:1 to enforce single-select
- base.js: lt.combobox Enter key only fired when focusedIdx >= 0;
  now falls back to first filtered result when no arrow key used
- DashboardView + dashboard.js + dashboard.css: add COLS ▾ button on
  table header that opens a checkbox panel to show/hide optional
  columns (Ticket ID, Category, Type, Created By, Assigned To,
  Created, Updated); state persisted in localStorage, Reset button
  restores all; core columns (Priority, Title, Status, Actions) always
  visible; data-col attributes added to all th/td for CSS targeting

Notifications bell: was functional all along — was broken by the
notifications.php 500 error (now fixed). Avg resolution: correct,
tickets genuinely take ~158 days average on this dataset.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/TicketView.php

@@ -64,9 +64,8 @@ function formatAction(array $event): string {
     }
 }
 
-// Calculate ticket age
-$lastUpdate  = !empty($ticket['updated_at']) ? strtotime($ticket['updated_at']) : strtotime($ticket['created_at']);
-$ageSeconds  = time() - $lastUpdate;
+// Calculate ticket age from creation (not last update)
+$ageSeconds  = time() - strtotime($ticket['created_at']);
 $ageDays     = floor($ageSeconds / 86400);
 $ageHours    = floor(($ageSeconds % 86400) / 3600);
 $ageClass    = 'lt-text-muted';