Epic: Expand integration test suite

[mjkatgithub]

Planning

• Milestone: Phase 4
• Priority: P2
• Size: L
• Estimate: 8+ SP (split into sub-issues)

Goal

Turn tests/integration/ from a thin smoke layer (3 files, 9 tests) into
meaningful module integration that catches wiring bugs between composables,
utils, and Vue components — without duplicating E2E or full unit mock suites.

Issue templates and README currently imply integration coverage per feature;
most issues today rely on unit + E2E only. This epic closes that gap.

Background

Current integration tests:

File	What it does today
useMatrixClient.spec.ts	Asserts exported function types exist
appI18n.spec.ts	One mounted component + translateText
chatTimeline.spec.ts	Timeline mapping with mocked Matrix room events

These are useful but not full integration (no login flow wiring, no
composable chains, no sidebar + unread pipeline).

Follow-up to #122 (CI full lane green) and README "Test layers" section.

Scope (sub-issues to create)

• Matrix client integration — login/session restore chain with shared
  mocks; send message → timeline mapping round-trip
• Chat UI integration — mount key components with real composable
  stubs (MessageList + timeline utils, SpaceHomePanel + hierarchy utils)
• i18n + component integration — expand beyond single string; DE/EN
  switch in mounted flows used by chat/auth
• Integration test conventions doc — when to use integration vs unit
  vs E2E; file naming; shared fixtures under tests/integration/support/

Definition of Done

• All sub-issues created and linked under this epic
• tests/integration/ has ≥ 25 meaningful tests across ≥ 6 spec files
• README "Test layers" updated with integration examples
• test:integration remains fast enough for CI fast lane (< 30s target)

Nicht im Scope

• @nuxt/test-utils / Nitro route tests (separate future epic)
• Replacing E2E scenarios with integration tests
• Matrix live homeserver in integration (mocks/fixtures only)

Notes

• Prefer real module imports + narrow mocks at boundaries (Matrix SDK,
  useState) over re-testing unit cases.
• Keep each spec file ≤ 800 lines (CCD).
• Docs: update docs/architecture.md test row when conventions land.

Related

• CI: Fix full lane (coverage + full E2E) #122 — CI lanes; integration runs in fast + full lane
• Refactor: Oversized modules splitten + Clean Code Developer Standards #63 — useMatrixClient split (mock helpers in unit tests reusable here)