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/js/base.js

@@ -1963,7 +1963,7 @@
       inputEl.addEventListener('keydown', e => {
         if (e.key === 'ArrowDown') { e.preventDefault(); _moveFocus(1); }
         if (e.key === 'ArrowUp')   { e.preventDefault(); _moveFocus(-1); }
-        if (e.key === 'Enter')     { e.preventDefault(); if (focusedIdx >= 0 && filtered[focusedIdx]) _toggle(filtered[focusedIdx].value); }
+        if (e.key === 'Enter')     { e.preventDefault(); const idx = focusedIdx >= 0 ? focusedIdx : 0; if (filtered[idx]) _toggle(filtered[idx].value); }
         if (e.key === 'Escape')    { _setOpen(false); }
         if (e.key === 'Backspace' && !inputEl.value && selected.length) { _toggle(selected[selected.length - 1]); }
       });