docs: add contract testing guide, service worker, error handling, and accessibility docs

[G-ELM]

Summary

Adds four documentation files closing gaps across the Soroban contracts and the web
frontend:

• contracts/docs/testing.md — the standard Soroban test scaffolding (Env::default(),
  Address::generate), the three levels of auth mocking (mock_all_auths,
  mock_all_auths_allowing_non_root_auth, and asserting real auth via env.auths() /
  mock_auths) and the risk of blanket mocking hiding a missing require_auth(), testing
  expiry via the virtual ledger clock, the proposals → group_treasury cross-contract
  test setup, and the test_snapshots/ directory (generated by cargo test, committed,
  Prettier-ignored).

• apps/web/docs/concepts-service-worker.md — sw.js registration/update lifecycle
  (skipWaiting + clients.claim), the content-free push handler (cross-linked to the
  backend push docs), click-to-route behaviour that focuses an existing tab instead of
  opening a duplicate, and a plain statement of what does and does not work offline today.

• apps/web/docs/concepts-error-handling.md — the toast API vs. inline error state,
  mapping backend error responses (REST schema error format, per-endpoint docs) to
  user-facing messages, the hard rule that decryption failures always render as
  UnavailableMessagePlaceholder rather than a generic crash, and current send-failure /
  offline behaviour including known gaps (silent socket errors, no automatic retry or
  offline queue).

• apps/web/docs/accessibility.md — states the WCAG 2.1 AA target and that it's
  currently checked manually; documents keyboard navigation through the conversation list
  and composer, contrasts Modal's full focus-trap/restore implementation against the
  safety-number panel's current gap, the aria-live approach for incoming messages that
  announces without stealing focus, a colour-contrast risk in low-opacity text utilities,
  and a pre-merge checklist for new interactive components.

All four are registered in docs/README.md. Several docs call out real, currently-existing
gaps (missing toast on send failure, un-trapped safety panel focus, no automated a11y
tooling) rather than describing aspirational behaviour, so the docs stay accurate.

Test plan

• Docs-only change; no code behavior modified. Reviewed for accuracy against the
  current implementation of sw.js, useToast.ts, Modal.tsx,
  ConversationListSidebar.tsx, MessageThread.tsx, and the three contracts' test
  suites.

Closes #577
Closes #578
Closes #574
Closes #575

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