fix: balance adjustment silently did nothing on a decimal comma

Pressing Apply on Settings -> Adjust balance did nothing at all: no error, no
request, nothing in the API logs. The cause is in code untouched since
062ba52 (2026-07-01), so this predates the recent releases — it just needs
the right input to show itself.

The form only renders when adjustingAccount?.id === account.id, so
adjustingAccount is never null while Apply is visible. That leaves exactly
one way for the handler to return silently:

    if (!adjustingAccount || !adjustTargetBalance) return

i.e. adjustTargetBalance === "". And the field was <input type="number">,
which reports value === "" for anything the browser judges invalid —
including a decimal comma. Every amount in this app is rendered with
Intl.NumberFormat('de-DE'), so typing 1234,56 is the natural thing to do, and
it left the state empty and the guard swallowed the click.

Fixes both halves:

- The input is now type="text" inputMode="decimal", so what the user types
  actually reaches the handler.
- The guard reports "Enter a target balance, e.g. 1234.56" instead of
  returning silently. A control that does nothing when pressed is the worst
  possible failure mode.

Adds parseDecimal() alongside parseAmount() in lib/utils.ts. Balance targets
differ from expense rows: zero and negative are legitimate, so parseAmount's
n > 0 rule would have been wrong here.

parseDecimal is also stricter than the parseFloat(s.replace(',', '.')) idiom
used across the flows, in two ways that matter on a balance field:

- "1.000,50" parsed as 1 under the old idiom — a silent 1000x error. It now
  reads the rightmost separator as the decimal point, giving 1000.50, and
  handles "1,000.50" the same way.
- "12.5abc" quietly became 12.5 under parseFloat. Number() rejects it.

parseAmount is redefined on top of parseDecimal, so the batch expense grid
picks up both improvements.

Verified: tsc + vite build clean; the 60 backend tests still pass (untouched);
parser checked against .-decimal, ,-decimal, both-separator, negative, zero,
whitespace and trailing-garbage inputs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L3qYLuu9jgXsCRXmJ18YGY

web/src/lib/utils.ts

@@ -10,9 +10,42 @@ export function todayLocalISO(): string {
   return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
 }
 
-// Accepts both 12.40 and the German 12,40. Returns null for anything that is
-// not a usable positive amount, so callers can report which row is wrong.
+// Accepts both 12.40 and the German 12,40, which is what the de-DE formatting
+// used throughout the app trains you to type. Returns null when the input is
+// not a number at all. Zero and negatives are valid — a target balance or a
+// payable can legitimately be either.
+//
+// Pair this with <input type="text" inputMode="decimal">, never
+// <input type="number">: a number input silently reports value === "" for
+// anything it considers invalid, including a decimal comma, so the bad input
+// never reaches the parser and the caller sees an empty field instead.
+export function parseDecimal(input: string): number | null {
+  const t = input.trim()
+  if (t === '') return null
+
+  const lastComma = t.lastIndexOf(',')
+  const lastDot = t.lastIndexOf('.')
+  let normalised: string
+  if (lastComma !== -1 && lastDot !== -1) {
+    // Both separators present, so the rightmost one is the decimal point and
+    // the other is thousands grouping. Without this, "1.000,50" would parse
+    // as 1 — a silent 1000x error on a balance field.
+    normalised = lastComma > lastDot
+      ? t.replace(/\./g, '').replace(',', '.')
+      : t.replace(/,/g, '')
+  } else {
+    normalised = t.replace(',', '.')
+  }
+
+  // Number() rather than parseFloat(): parseFloat stops at the first bad
+  // character, so "12.5abc" would quietly become 12.5.
+  const n = Number(normalised)
+  return Number.isFinite(n) ? n : null
+}
+
+// Same, but rejects zero and negatives — for fields that must be a positive
+// amount, so callers can report which row is wrong.
 export function parseAmount(input: string): number | null {
-  const n = parseFloat(input.replace(',', '.'))
-  return Number.isFinite(n) && n > 0 ? n : null
+  const n = parseDecimal(input)
+  return n !== null && n > 0 ? n : null
 }

web/src/pages/Settings.tsx

@@ -2,6 +2,7 @@ import { useState, useEffect } from 'react'
 import { useNavigate } from 'react-router-dom'
 import { ChevronLeft } from 'lucide-react'
 import { api, Account, Category } from '@/api/client'
+import { parseDecimal } from '@/lib/utils'
 
 export default function Settings() {
   const navigate = useNavigate()
@@ -90,10 +91,16 @@ export default function Settings() {
   }
 
   const saveAdjustBalance = async () => {
-    if (!adjustingAccount || !adjustTargetBalance) return
+    if (!adjustingAccount) return
+    // Zero and negative targets are legitimate, so parseDecimal, not parseAmount.
+    const target = parseDecimal(adjustTargetBalance)
+    if (target === null) {
+      setMutationError('Enter a target balance, e.g. 1234.56')
+      return
+    }
     setMutationError(null)
     try {
-      await api.adjustAccountBalance(adjustingAccount.id, parseFloat(adjustTargetBalance))
+      await api.adjustAccountBalance(adjustingAccount.id, target)
       setAdjustingAccount(null)
       setAdjustTargetBalance('')
       window.dispatchEvent(new CustomEvent('fin:transaction-added'))
@@ -263,8 +270,8 @@ export default function Settings() {
                   value={adjustTargetBalance}
                   onChange={e => setAdjustTargetBalance(e.target.value)}
                   placeholder="Target balance"
-                  type="number"
-                  step="0.01"
+                  type="text"
+                  inputMode="decimal"
                 />
                 <div className="flex gap-2">
                   <button