feat(costs): rework Budget into Costs — Splitwise-style, multi-currency, mobile (#1106)

* fix(journey): authorize reads of the journey share link

GET /api/journeys/:id/share-link now requires journey access (canAccessJourney),
matching the create/delete share-link routes and the get_journey_share_link MCP
tool. Returns no link when the caller lacks access to the journey.

* feat(costs): rework Budget into Costs — Splitwise-style, multi-currency, mobile

Renames the Budget addon to "Costs" (UI only) and reworks it into a Tricount/
Splitwise-style cost tracker: multiple payers per expense, equal split across
chosen members, settle-up with persisted history + undo, 12 fixed categories,
per-expense currency with live FX conversion to a user-set display currency
(Settings -> Display), and locale-correct money formatting. Adds a desktop and a
dedicated mobile layout. A migration backfills existing budget items (single
payer, split members, currency). Closes #551 (per-expense currency).

Also switches the app font to self-hosted Poppins (Geist for secondary subtext),
replacing the Google Fonts CDN dependency.

* fix(costs): neutral dashboard dark palette + liquid glass, full page width, entry-count badge

- Dark mode used a warm oklch palette that read brownish; switch to the
  neutral zinc tokens used by the dashboard (#121215 bg, #f4f4f5 ink) and add a
  subtle backdrop-blur glass on cards.
- Costs now uses the full available page width on desktop instead of a 1280px cap.
- Render the expense count next to the Expenses title as a badge.
- Adapt budget/journey unit tests to the new payer-based settlement model and the
  Costs rename (category default 'other', Costs tab/CostsPanel).

* fix(costs): drop the entry-count badge, always show row edit/delete actions

Removes the count badge next to the Expenses title and makes the per-row
edit/delete actions permanently visible (no longer hover-only) on desktop too.

* feat(costs): currency-native money formatting, custom select/date, rename addon to Costs

- Format every amount in its own currency convention (symbol position, grouping
  and decimal separators) regardless of app language, via a currency->locale map
  (EUR -> '12,00 €', USD -> '$12.00', JPY -> '¥12', ...). Previously Intl used the
  app locale, so EUR showed the symbol in front under an English UI.
- Use TREK's CustomSelect (searchable, with symbols) and CustomDatePicker in the
  add/edit expense modal instead of the native <select>/<input type=date>.
- Rename the 'Budget Planner' add-on to 'Costs' in the admin list (display only;
  id/tables/permissions/MCP stay 'budget') via seed + a migration for existing DBs.

* feat(auth): configurable session duration via SESSION_DURATION

Adds a SESSION_DURATION env var (ms-style strings: 1h, 7d, 30d, ...) controlling
how long a session stays valid before re-login. It drives both the trek_session
JWT exp claim and the cookie maxAge from one source, so they never drift. Invalid
values warn at startup and fall back to the default (24h — unchanged). The MFA
challenge token and MCP OAuth tokens keep their own TTL.

Implements the request from discussion #946. Documented in the env-var wiki page,
.env.example and docker-compose.yml.

client/src/utils/formatters.ts

@@ -39,6 +39,58 @@ export function currencyDecimals(currency: string): number {
   return ZERO_DECIMAL_CURRENCIES.has(currency.toUpperCase()) ? 0 : 2
 }
 
+// Each currency formats in its own home convention (symbol position, grouping and
+// decimal separators) regardless of the app language — so EUR is always "1.234,56 €"
+// and USD always "$1,234.56". Intl derives all of that from the locale, so we map
+// each supported currency to a representative locale (Latin-digit variants for the
+// Arabic/Bengali ones to avoid non-Latin numerals).
+const CURRENCY_LOCALE: Record<string, string> = {
+  EUR: 'de-DE', USD: 'en-US', GBP: 'en-GB', JPY: 'ja-JP', CHF: 'de-CH',
+  CZK: 'cs-CZ', PLN: 'pl-PL', SEK: 'sv-SE', NOK: 'nb-NO', DKK: 'da-DK',
+  TRY: 'tr-TR', THB: 'th-TH', AUD: 'en-AU', CAD: 'en-CA', NZD: 'en-NZ',
+  BRL: 'pt-BR', MXN: 'es-MX', INR: 'en-IN', IDR: 'id-ID', MYR: 'ms-MY',
+  PHP: 'en-PH', SGD: 'en-SG', KRW: 'ko-KR', CNY: 'zh-CN', HKD: 'en-HK',
+  TWD: 'zh-TW', ZAR: 'en-ZA', AED: 'en-AE', SAR: 'en-SA', ILS: 'he-IL',
+  EGP: 'en-EG', MAD: 'fr-MA', HUF: 'hu-HU', RON: 'ro-RO', BGN: 'bg-BG',
+  HRK: 'hr-HR', ISK: 'is-IS', RUB: 'ru-RU', UAH: 'uk-UA', BDT: 'en-BD',
+  LKR: 'en-LK', VND: 'vi-VN', CLP: 'es-CL', COP: 'es-CO', PEN: 'es-PE',
+  ARS: 'es-AR',
+}
+
+export function currencyLocale(currency: string): string {
+  return CURRENCY_LOCALE[(currency || '').toUpperCase()] || 'en-US'
+}
+
+/**
+ * Locale- and currency-correct money formatting via Intl: the symbol position,
+ * thousands/decimal separators and decimal count all follow the user's locale
+ * and the currency itself (e.g. de-DE EUR → "1.234,56 €", en-US USD → "$1,234.56",
+ * ja-JP JPY → "￥1,235"). Falls back to a "<number> CODE" suffix for unknown codes.
+ */
+export function formatMoney(
+  value: number,
+  currency: string,
+  locale: string,
+  opts?: { decimals?: number },
+): string {
+  const cur = (currency || 'EUR').toUpperCase()
+  const decimals = opts?.decimals ?? currencyDecimals(cur)
+  // Format in the currency's home convention, not the app language, so the symbol
+  // position and separators are always correct for that currency. `locale` stays
+  // as a last-resort fallback for the error path.
+  const fmtLocale = currencyLocale(cur)
+  try {
+    return new Intl.NumberFormat(fmtLocale, {
+      style: 'currency',
+      currency: cur,
+      minimumFractionDigits: decimals,
+      maximumFractionDigits: decimals,
+    }).format(value || 0)
+  } catch {
+    return `${(value || 0).toLocaleString(locale || fmtLocale, { minimumFractionDigits: decimals, maximumFractionDigits: decimals })} ${cur}`
+  }
+}
+
 export function formatDate(dateStr: string | null | undefined, locale: string, timeZone?: string): string | null {
   if (!dateStr) return null
   const opts: Intl.DateTimeFormatOptions = {