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/index.css

@@ -431,7 +431,9 @@ input[type="number"], input[type="time"], input[type="date"], input[type="dateti
   --safe-top: env(safe-area-inset-top, 0px);
   --nav-h: 0px;
   --bottom-nav-h: 0px;
-  --font-system: -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'Segoe UI', system-ui, sans-serif;
+  --font-system: 'Poppins', -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'Segoe UI', system-ui, sans-serif;
+  /* Secondary "subtext"/caption tier renders in Geist; primary text + headings stay Poppins. */
+  --font-subtext: 'Geist Sans', 'Poppins', -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
   --sp-1: 4px;
   --sp-2: 8px;
   --sp-3: 12px;
@@ -539,6 +541,11 @@ body {
   transition: background-color 0.2s, color 0.2s;
 }
 
+/* Subtext tier in Geist. The faint text token is TREK's caption/secondary tier;
+   a direct rule on the element beats the Poppins inherited from wrapper styles,
+   giving the design's "Geist text · Poppins numbers" hierarchy. */
+.text-content-faint { font-family: var(--font-subtext); }
+
 /* ── Marker cluster custom styling ────────────── */
 .marker-cluster-wrapper {
   background: transparent !important;
@@ -563,7 +570,7 @@ body {
 }
 
 .marker-cluster-custom span {
-  font-family: -apple-system, BlinkMacSystemFont, 'SF Pro Text', system-ui, sans-serif;
+  font-family:var(--font-system);
   font-size: 12px;
   font-weight: 700;
   color: #ffffff;