mobile: document mock mode indicators

[duct-tape2]

Goal

Document exactly how the mobile app signals fixture-backed mock mode versus API-backed real mode, including the important distinction between the selected mode and actual backend health.

Scope

• Edit only docs/ARCHITECTURE.md.
• Add a short subsection near Runtime data flow or the mobile module map.
• Explain the source-of-truth switch: USE_MOCK is true unless EXPO_PUBLIC_USE_MOCK_API=false.
• Document these visible indicators:
  • Daily Talk shows STRINGS.dailyTalk.demoBadge in mock mode and STRINGS.dailyTalk.liveBadge in real mode.
  • Courses shows 학습 코스 (오프라인 미리보기) in mock mode.
  • Settings shows the raw API mode, base URL, and health check only when SHOW_DEV_TOOLS is enabled in a development build; this panel is not an end-user production indicator.
• Document the behavior behind the labels:
  • Mock mode returns shared fixtures immediately and loads the bundled dialogue pack, with device TTS used when fixture audio is absent.
  • Real mode attempts API_BASE. Read-style calls may use their checked-in fixture fallback after network or HTTP failure.
  • Mutations or personal-data calls marked noFallbackInReal surface the error instead of pretending fixture-backed success.
• State explicitly that the liveBadge reports the selected client mode; it does not by itself prove the backend or a local STT/TTS engine is healthy. The development health check is the separate diagnostic.
• Use apps/mobile/src/api/client.ts, apps/mobile/src/devConfig.ts, apps/mobile/src/screens/DailyTalkScreen.tsx, apps/mobile/src/screens/CoursesScreen.tsx, and apps/mobile/src/screens/SettingsScreen.tsx as read-only sources of truth.
• Do not change application code, translations, screenshots, generated Pages files, or other docs.

Acceptance Criteria

• A small table or equivalent compact section distinguishes mock, real with healthy API, and real with a read fallback.
• Every documented label and fallback statement matches current source behavior.
• The documentation does not imply that selecting real mode guarantees a live speech engine.
• The PR changes only docs/ARCHITECTURE.md and includes Closes #16.

Suggested Verification

This is a browser-editable docs task. No Expo, backend, speech engine, or API key is required.

python3 scripts/check_public_tree.py
git diff --check

Direct edit link:
https://github.com/duct-tape2/ai-language-partner/edit/main/docs/ARCHITECTURE.md

[github-actions]

Edit this issue in your browser

No local app, backend, speech engine, private data, or API key is required.

Change

Document how fixture-backed mock mode differs from live API-backed mode

Edit

• File: docs/ARCHITECTURE.md
• Open the direct edit page

Finish

• Comment /claim, make only this change, and write Closes #16 in
  the PR body.
• Use GitHub preview and name the wording, links, or examples you reviewed; no CLI check is required.
• Do not add generated media, archives, databases, screenshots, secrets, local
  engines, or private data. Do not split trivial changes across PRs.

Help

• Five-minute browser guide
• Run checks in Codespaces
• Ask the first PR help desk

[github-actions]

First PR recipe

Keep this to one useful change for this issue.

Inspect

• docs/ARCHITECTURE.md
• docs/ja/index.md
• docs/ko/index.md

Done when

• A small table or equivalent compact section distinguishes mock, real with healthy API, and real with a read fallback.
• Every documented label and fallback statement matches current source behavior.
• The documentation does not imply that selecting real mode guarantees a live speech engine.
• The PR changes only docs/ARCHITECTURE.md and includes Closes #16.

Verify

• python3 scripts/check_public_tree.py
• cd apps/mobile && npm run verify

Open the PR

• Write Closes #16 in the PR body.
• Say what changed and name the check or review you completed.
• Do not add generated media, archives, local engines, databases, secrets,
  screenshots, or private data.

Need a route?

• Browser edit: https://duct-tape2.github.io/ai-language-partner/community/FIVE_MINUTE_FIRST_PR.html
• Code or tests without local setup: https://duct-tape2.github.io/ai-language-partner/community/CODESPACES_FIRST_PR.html
• Ask a maintainer: First PR help desk: pick a useful starter issue #53