feat: E2E test coverage with Detox

[jamilagibson]

Context

Current test coverage:

• Unit tests — Vitest + RTL (components, utilities, Redux slices)
• Integration tests — real WebSocket server, full RN → server → Redux round-trip

Device-level behavior is not covered: native gesture handling, platform-specific rendering, physical WebSocket connections on a real device or simulator.

Scope

• App launch and initial WebSocket connection
• State change triggers a snapshot emission
• Browser UI receives and displays the snapshot
• Timeline scrubber navigates to a previous snapshot

Acceptance criteria

• Detox configured for iOS simulator and Android emulator
• Core user journey covered: launch → interact → observe in debugger
• Tests run in CI
• README.md documents how to run E2E tests locally