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/Views/Inventory/Ledger.cshtml

@@ -353,6 +353,11 @@
                         <label class="form-label fw-semibold">Powder Item</label>
                         <p id="euItemName" class="form-control-plaintext text-muted"></p>
                     </div>
+                    <div class="mb-3">
+                        <label for="euQuantity" class="form-label fw-semibold">Amount Used <small class="text-muted fw-normal" id="euQuantityUom"></small></label>
+                        <input type="number" id="euQuantity" name="quantity" class="form-control" min="0.001" step="any" required />
+                        <div class="form-text">Adjusts the inventory balance by the difference from the original entry.</div>
+                    </div>
                     <div class="mb-3">
                         <label for="euJobId" class="form-label fw-semibold">Job <span class="text-muted fw-normal">(optional)</span></label>
                         <select id="euJobId" name="jobId" class="form-select">