feat: Read/query entrypoints: get_policy, get_claim, bounded pagination

[Macnelson9]

This PR closes #19

feat(contract): read/query entrypoints with bounded pagination

Add get_policy, get_claim, list_policies, and list_claims to the
Soroban contract, plus matching frontend chain-read helpers.

Contract changes

• list_policies(holder, start_after, limit) — paginated policy
  summaries, exclusive cursor over policy_id, limit hard-capped at 20
  (PAGE_SIZE_MAX)
• list_claims(start_after, limit) — same pattern over claim_id
• PolicySummary / ClaimSummary — lightweight structs (omit details,
  image_urls) to keep per-page byte cost predictable; callers follow
  up with get_policy / get_claim for full records
• Stale cursors return an empty page — never panic, never skip
  records (IDs are monotonically increasing, never deleted)

Tests — 10 new cases in tests/storage.rs: empty page, first page,
cursor advance, last partial page, cursor-past-end, oversize limit
clamped to 20

Frontend changes

• src/lib/api/chain.ts — new module: getPolicy, getClaim,
  listPolicies, listClaims via Soroban simulation; null return on 404
  for predictable unknown-id handling
• src/types/claim.ts — OnChainPolicySummary, OnChainClaimSummary,
  CHAIN_PAGE_SIZE_MAX; documents chain-read vs indexer-read tradeoffs
  (lag, finality, trust, cost)

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