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.

docker-compose.yml

@@ -22,6 +22,7 @@ services:
       - TZ=${TZ:-UTC} # Timezone for logs, reminders and scheduled tasks (e.g. Europe/Berlin)
       - LOG_LEVEL=${LOG_LEVEL:-info} # info = concise user actions; debug = verbose admin-level details
 #      - DEFAULT_LANGUAGE=en # Default language on the login page for users with no saved preference. Browser/OS language is auto-detected first; this is the fallback. Supported: de, en, es, fr, hu, nl, br, cs, pl, ru, zh, zh-TW, it, ar
+#      - SESSION_DURATION=30d # How long users stay logged in (trek_session JWT + cookie maxAge). Accepts: 1h | 12h | 7d | 30d | 90d. Default: 24h
       - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-} # Comma-separated origins for CORS and email notification links
 #      - FORCE_HTTPS=true # Optional. Enables HTTPS redirect, HSTS, CSP upgrade-insecure-requests, and secure cookies behind a TLS proxy
 #      - HSTS_INCLUDE_SUBDOMAINS=false # When true: adds includeSubDomains to the HSTS header. Only effective when HSTS is active. Leave false if sibling subdomains still run over plain HTTP.