Reporting page: add Due Date, column manager (hide/reorder), remove Discovered/Source

Backend:
- Extract dueDate from statusEmbedded.dueDate (strip time portion)
- Remove discoveredOn and source from extractFinding (not needed)

Frontend:
- Add Due Date column (color-coded: red=past due, amber=within 30d, gray=future)
- Remove Discovered and Source columns
- ColumnManager component: gear button opens popover with drag-to-reorder and
  eye toggle per column; column state persisted to localStorage
- Column order/visibility survives page refresh and syncs
- SortIcon, TableCell, NoteCell all driven by current visible column list

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

backend/routes/ivantiFindings.js

@@ -120,6 +120,10 @@ function initTables(db) {
 // Extract only the fields we need from a raw finding object
 // ---------------------------------------------------------------------------
 function extractFinding(f) {
+    // statusEmbedded.dueDate = "2026-03-06T00:00:00" — strip to date part
+    const rawDueDate = f.statusEmbedded?.dueDate || '';
+    const dueDate = rawDueDate ? rawDueDate.split('T')[0] : '';
+
     return {
         id: String(f.id),
         title: f.title || '',
@@ -130,11 +134,8 @@ function extractFinding(f) {
         dns: f.dns || f.host?.fqdn || '',
         status: f.status || '',
         slaStatus: f.slaStatus || '',
-        discoveredOn: f.discoveredOn || '',
-        lastFoundOn: f.lastFoundOn || '',
-        source: f.scannerPrettyName || f.scannerName || f.source || '',
-        pluginFamily: f.pluginFamily || '',
-        findingType: f.findingType || ''
+        dueDate,
+        lastFoundOn: f.lastFoundOn || ''
     };
 }