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/nest/budget/budget.service.ts

@@ -4,6 +4,7 @@ import { broadcast } from '../../websocket';
 import { checkPermission } from '../../services/permissions';
 import type { User } from '../../types';
 import * as svc from '../../services/budgetService';
+import { getRates } from '../../services/exchangeRateService';
 
 type Trip = NonNullable<ReturnType<typeof svc.verifyTripAccess>>;
 
@@ -34,8 +35,10 @@ export class BudgetService {
     return svc.getPerPersonSummary(tripId);
   }
 
-  settlement(tripId: string) {
-    return svc.calculateSettlement(tripId);
+  async settlement(tripId: string, base: string | undefined, tripCurrency: string) {
+    const effectiveBase = (base || tripCurrency || 'EUR').toUpperCase();
+    const rates = await getRates(effectiveBase);
+    return svc.calculateSettlement(tripId, { base: effectiveBase, rates, tripCurrency });
   }
 
   create(tripId: string, data: Parameters<typeof svc.createBudgetItem>[1]) {
@@ -58,6 +61,22 @@ export class BudgetService {
     return svc.toggleMemberPaid(id, userId, paid);
   }
 
+  setPayers(id: string, tripId: string, payers: { user_id: number; amount: number }[]) {
+    return svc.setItemPayers(id, tripId, payers);
+  }
+
+  listSettlements(tripId: string) {
+    return svc.listSettlements(tripId);
+  }
+
+  createSettlement(tripId: string, data: { from_user_id: number; to_user_id: number; amount: number }, userId: number) {
+    return svc.createSettlement(tripId, data, userId);
+  }
+
+  deleteSettlement(id: string, tripId: string): boolean {
+    return svc.deleteSettlement(id, tripId);
+  }
+
   reorderItems(tripId: string, orderedIds: number[]): void {
     svc.reorderBudgetItems(tripId, orderedIds);
   }