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.

server/src/services/exchangeRateService.ts

@@ -0,0 +1,68 @@
+/**
+ * Live exchange rates for the Costs/Budget money conversion.
+ *
+ * Fetches from exchangerate-api.com (no key, already CSP-allowlisted for the
+ * dashboard widget) and caches per base currency in-memory for a few hours so a
+ * settlement request never hammers the upstream. Rates are "units of X per 1
+ * base", so an amount in currency C converts to base as `amount / rates[C]`.
+ *
+ * Everything degrades gracefully: if the fetch fails (offline, upstream down),
+ * callers get `null`/identity conversion and amounts are treated as already in
+ * the base currency rather than throwing.
+ */
+
+const TTL_MS = 6 * 60 * 60 * 1000; // 6h
+const cache = new Map<string, { rates: Record<string, number>; ts: number }>();
+const inflight = new Map<string, Promise<Record<string, number> | null>>();
+
+async function fetchRates(base: string): Promise<Record<string, number> | null> {
+  try {
+    const res = await fetch(`https://api.exchangerate-api.com/v4/latest/${encodeURIComponent(base)}`);
+    if (!res.ok) return null;
+    const data = (await res.json()) as { rates?: Record<string, number> };
+    return data.rates && typeof data.rates === 'object' ? data.rates : null;
+  } catch {
+    return null;
+  }
+}
+
+/** Rates map for `base` (cached). Returns null if unavailable. */
+export async function getRates(base: string): Promise<Record<string, number> | null> {
+  const key = (base || 'EUR').toUpperCase();
+  const hit = cache.get(key);
+  const now = Date.now();
+  if (hit && now - hit.ts < TTL_MS) return hit.rates;
+
+  // Coalesce concurrent fetches for the same base.
+  let p = inflight.get(key);
+  if (!p) {
+    p = fetchRates(key).then(rates => {
+      if (rates) cache.set(key, { rates, ts: Date.now() });
+      inflight.delete(key);
+      return rates;
+    });
+    inflight.set(key, p);
+  }
+  const rates = await p;
+  // On failure fall back to the last cached value if we have one.
+  if (!rates && hit) return hit.rates;
+  return rates;
+}
+
+/**
+ * Convert `amount` from `from` currency into `base` using a rates map obtained
+ * from getRates(base). Identity when same currency or the rate is missing.
+ */
+export function convertWithRates(
+  amount: number,
+  from: string | null | undefined,
+  base: string,
+  rates: Record<string, number> | null,
+): number {
+  const fromCur = (from || base).toUpperCase();
+  const baseCur = base.toUpperCase();
+  if (fromCur === baseCur || !rates) return amount;
+  const r = rates[fromCur];
+  if (!r || r <= 0) return amount;
+  return amount / r;
+}