feat: add campaign discovery / marketplace list at /campaigns

[LamsOfJos]

Campaign discovery / marketplace list (#142)

CampaignsPage (pages/index.tsx) was a static PlaceholderPage — the app
had no page anywhere for an investor to browse available campaigns. Every
other flow (funding, detail, dashboard) assumed you already had a campaign id.
This adds a real, live list at /campaigns.

Changes

File	What
hooks/useAllCampaigns.ts (new)	Discovers every campaign from ProductionEscrowContract event history and fetches current state via get_campaign. Same event-scan approach as useAdminCampaigns.ts but without the ACTIONABLE_STATUSES admin filter; sorted newest-id-first, guarded by isEscrowConfigured(), unresolvable ids skipped instead of failing the whole load.
pages/CampaignsPage.tsx (new)	Real list page. Card grid where each card shows <StatusBadge> + a role="progressbar" funding bar and links to /campaigns/:id. Handles loading (CampaignCardsSkeleton), error (message + Retry via refetch), empty (no campaigns yet + create CTA), and not-configured (env-var hint) states. Status filter chips built on STATUS_META, shown only when >1 status is present.
App.tsx	<Route path="/campaigns" element={<CampaignsPage />} /> added to the AppLayout group (nav already linked there).
pages/index.tsx	Placeholder CampaignsPage removed; now export { CampaignsPage } from './CampaignsPage', matching the ActivityFeedPage/FarmerProfilePage pattern.
hooks/contract/queryKeys.ts	Added allCampaigns query key.
__tests__/CampaignsPage.test.tsx (new)	Asserts the list renders with correct /campaigns/:id hrefs, plus the empty / error / loading states.

Acceptance criteria

• /campaigns renders a real, live list of on-chain campaigns, not placeholder copy
• Each campaign card links to a working /campaigns/:id detail page
• Empty (no campaigns yet) and error (RPC/backend failure) states both handled

Verification

• tsc --noEmit — clean
• eslint — clean on all touched files
• Full test suite — 88/88 pass (20 files), including 4 new page tests

Closes #142