core: implement React Query cache architecture with optimistic updates

[EmeditWeb]

Problem

Pages fetch data independently with ad-hoc query usage and no shared cache
strategy. After a mutation (deposit, vouch, product CRUD) other views keep
showing stale data until a manual refresh, there are no optimistic updates
or rollback, and query keys are inconsistent across services. This produces
visible inconsistencies (e.g. a new product not appearing on the vendor
detail page, pool stats not updating after a deposit).

Before Starting

Read ALL of these before writing any code:

• context/architecture-context.md
• context/code-standards.md

What To Build

A React Query cache architecture standardized across the services layer:

1. A central query-key factory so every entity (pool, loans, vouches,
   vendors, reputation) has canonical, hierarchical keys.
2. A mutation → invalidation map so each mutation invalidates exactly the
   affected query subtrees on success.
3. An optimistic-update pattern with rollback for the high-frequency
   mutations (vouch accept/decline, product CRUD, deposit/withdraw).
4. Sensible staleTime / gcTime defaults and a documented convention that
   new services must follow.

Files To Touch

• src/services/queryKeys.ts (new)
• src/services/*.service.ts (adopt keys + invalidation)
• src/hooks/* (optimistic mutation helpers)
• relevant page components

Acceptance Criteria

• Canonical query-key factory used by all services
• Mutations invalidate the correct query subtrees on success
• Optimistic updates with rollback on the listed high-frequency mutations
• Stale data no longer shown across views after a mutation
• Tests cover invalidation and optimistic rollback behavior

Mandatory Checks Before Opening PR

• All context/ files read and understood
• npm run lint passes with zero errors
• npm run build passes with zero errors
• npm test passes — test count not decreased
• No hardcoded hex colors; icons from lucide-react only
• No API calls in page files
• Tested on 375px mobile viewport
• PR references this issue number