Fix material usage logging: remaining weight mode, edit modal, and consolidate duplicate logic

- InventoryController: extract RecordInventoryUsageAsync helper; both LogUsage
  (scan page) and LogMaterial (jobs modal, moved from JobsController) call it —
  no more duplicate save/GL logic across two controllers
- Log Material modal: replace radio buttons with prominent toggle buttons so the
  active mode (Amount Used vs Amount Remaining) is always visually obvious; add
  always-visible preview line showing exactly what will be logged before saving
- Edit Usage modal: add quantity field (pre-populated from existing transaction)
  with delta adjustment to InventoryItem.QuantityOnHand on save; include
  completed/terminal jobs in the dropdown so entries can be corrected after a
  job is marked done
- Scan page job picker: include jobs completed within the last 7 days (marked
  with '(completed)') so usage can be logged after a job is finished

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

src/PowderCoating.Web/wwwroot/js/inventory-ledger.js

@@ -18,6 +18,7 @@ async function openUsageEdit(transactionId) {
 
         document.getElementById('euTxnId').value = data.transactionId;
         document.getElementById('euItemName').textContent = data.itemName || '—';
+        document.getElementById('euQuantity').value = data.quantity != null ? parseFloat(data.quantity).toFixed(4) : '';
         document.getElementById('euDate').value = data.transactionDate;
         document.getElementById('euNotes').value = data.notes || '';
 
@@ -54,6 +55,7 @@ document.getElementById('euSaveBtn').addEventListener('click', async () => {
     const token = form.querySelector('input[name="__RequestVerificationToken"]')?.value;
     const params = new URLSearchParams({
         id: document.getElementById('euTxnId').value,
+        quantity: document.getElementById('euQuantity').value,
         jobId: document.getElementById('euJobId').value,
         notes: document.getElementById('euNotes').value,
         transactionDate: document.getElementById('euDate').value,