feat(01-02): implement InlineEditCell shared component

- Shared TableCell with display/edit mode toggle
- Calls onSave(number) on blur or Enter key
- Skips save when value unchanged or NaN
- Accepts className for amount coloring

frontend/src/components/InlineEditCell.tsx

@@ -0,0 +1,60 @@
+import { useState } from 'react'
+import { TableCell } from '@/components/ui/table'
+import { Input } from '@/components/ui/input'
+import { formatCurrency } from '@/lib/format'
+import { cn } from '@/lib/utils'
+
+interface InlineEditCellProps {
+  value: number
+  currency: string
+  onSave: (value: number) => Promise<void>
+  className?: string
+}
+
+export function InlineEditCell({ value, currency, onSave, className }: InlineEditCellProps) {
+  const [editing, setEditing] = useState(false)
+  const [inputValue, setInputValue] = useState(String(value))
+
+  const handleClick = () => {
+    setInputValue(String(value))
+    setEditing(true)
+  }
+
+  const handleBlur = async () => {
+    const num = parseFloat(inputValue)
+    if (!isNaN(num) && num !== value) {
+      await onSave(num)
+    }
+    setEditing(false)
+  }
+
+  const handleKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
+    if (e.key === 'Enter') {
+      handleBlur()
+    }
+  }
+
+  return (
+    <TableCell className={cn('text-right', className)}>
+      {editing ? (
+        <Input
+          type="number"
+          step="0.01"
+          value={inputValue}
+          onChange={(e) => setInputValue(e.target.value)}
+          onBlur={handleBlur}
+          onKeyDown={handleKeyDown}
+          className="ml-auto w-28 text-right"
+          autoFocus
+        />
+      ) : (
+        <span
+          className="cursor-pointer rounded px-2 py-1 hover:bg-muted"
+          onClick={handleClick}
+        >
+          {formatCurrency(value, currency)}
+        </span>
+      )}
+    </TableCell>
+  )
+}