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>

assets/css/dashboard.css

@@ -8,6 +8,43 @@
   margin-bottom: 1rem;
 }
 
+/* ── Column toggle panel ─────────────────────────────────────── */
+.col-toggle-panel {
+  position: absolute;
+  top: calc(100% + 4px);
+  right: 0;
+  z-index: 200;
+  background: var(--bg-card);
+  border: 1px solid var(--border-color);
+  border-radius: 4px;
+  min-width: 160px;
+  box-shadow: 0 4px 16px rgba(0,0,0,0.4);
+  display: none;
+}
+.col-toggle-panel[aria-hidden="false"] { display: block; }
+.col-toggle-title {
+  font-size: 0.6rem;
+  letter-spacing: 0.08em;
+  color: var(--text-muted);
+  padding: 0.4rem 0.65rem 0.25rem;
+  border-bottom: 1px solid var(--border-dim);
+  text-transform: uppercase;
+}
+.col-toggle-row {
+  display: flex;
+  align-items: center;
+  gap: 0.45rem;
+  padding: 0.3rem 0.65rem;
+  font-size: 0.72rem;
+  cursor: pointer;
+  transition: background 0.1s;
+}
+.col-toggle-row:hover { background: var(--bg-hover); }
+.col-toggle-footer {
+  padding: 0.3rem 0.45rem;
+  border-top: 1px solid var(--border-dim);
+}
+
 /* Unit suffix on resolution time stat (smaller, muted) */
 .lt-stat-unit {
   font-size: 0.65em;