feat(03-01): enhance InlineEditCell with pencil icon hover affordance and save/error callbacks

- Add Pencil icon in display mode (opacity-0, group-hover:opacity-100)
- Add onSaveSuccess optional callback fired after successful save
- Add onSaveError optional callback fired + value reverted on save failure
- Wrap onSave in try/catch in handleBlur

frontend/src/components/InlineEditCell.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react'
+import { Pencil } from 'lucide-react'
 import { TableCell } from '@/components/ui/table'
 import { Input } from '@/components/ui/input'
 import { formatCurrency } from '@/lib/format'
@@ -8,10 +9,12 @@ interface InlineEditCellProps {
   value: number
   currency: string
   onSave: (value: number) => Promise<void>
+  onSaveSuccess?: () => void
+  onSaveError?: () => void
   className?: string
 }
 
-export function InlineEditCell({ value, currency, onSave, className }: InlineEditCellProps) {
+export function InlineEditCell({ value, currency, onSave, onSaveSuccess, onSaveError, className }: InlineEditCellProps) {
   const [editing, setEditing] = useState(false)
   const [inputValue, setInputValue] = useState(String(value))
 
@@ -23,7 +26,13 @@ export function InlineEditCell({ value, currency, onSave, className }: InlineEdi
   const handleBlur = async () => {
     const num = parseFloat(inputValue)
     if (!isNaN(num) && num !== value) {
+      try {
         await onSave(num)
+        onSaveSuccess?.()
+      } catch {
+        setInputValue(String(value))
+        onSaveError?.()
+      }
     }
     setEditing(false)
   }
@@ -49,10 +58,14 @@ export function InlineEditCell({ value, currency, onSave, className }: InlineEdi
         />
       ) : (
         <span
-          className="cursor-pointer rounded px-2 py-1 hover:bg-muted"
+          className="group flex cursor-pointer items-center justify-end gap-1 rounded px-2 py-1 hover:bg-muted"
           onClick={handleClick}
         >
           {formatCurrency(value, currency)}
+          <Pencil
+            data-testid="pencil-icon"
+            className="size-3 opacity-0 transition-opacity duration-150 group-hover:opacity-100 text-muted-foreground"
+          />
         </span>
       )}
     </TableCell>