Merge pull request '[Need-to-have] #175 Shared i18n contract docs + bilingual MVP flow smoke' (#218) from dev/issue-175-shared-i18n-mainline into main

docs/I18N_ARCHITECTURE.md

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+# Shared i18n architecture (frontend + backend)
+
+## Scope
+Issue #175 requires one shared i18n contract for MVP host/player flows across frontend and backend.
+
+## Source of truth
+- Catalog: `shared/i18n/lobby.json`
+- Locales:
+  - supported: `en`, `da`
+  - default/fallback: `en`
+
+Both Angular (`frontend/angular/src/app/lobby-i18n.ts`) and Django (`lobby/i18n.py`) read from this catalog.
+
+## Key naming convention
+- Domain-first namespaces:
+  - `frontend.ui.common.*`
+  - `frontend.ui.host.*`
+  - `frontend.ui.player.*`
+  - `frontend.errors.*`
+  - `backend.errors.*`
+  - `backend.error_codes.*`
+- UI lookup keys in Angular are shortened aliases mapped under `frontend.ui`, e.g.:
+  - `host.start_round`
+  - `player.submit_guess`
+  - `common.session_code`
+- Backend API errors return stable code + localized message:
+  - `error_code` = machine-stable key from `backend.error_codes`
+  - `error` = localized message from `backend.errors`
+  - `locale` = resolved request locale
+
+## Fallback model (robust)
+1. Resolve requested locale (`Accept-Language` on backend, user/browser preference on frontend).
+2. If locale unsupported -> use default `en`.
+3. If key missing -> return key and log warning.
+4. If locale translation missing for key -> fallback to `en` translation.
+
+## Audio-routing policy
+- Catalog capability: `frontend.capabilities.client_has_no_audio_output = true`
+- Host/player clients expose this as a read-only capability flag.
+- Policy: phone clients must not play audio directly; only primary/host output is allowed.
+
+## Verification
+- Backend tests: `lobby/tests.py` i18n coverage for locale selection + fallback + error-code/message matrix.
+- Frontend smoke/e2e-level unit coverage:
+  - `frontend/angular/src/app/lobby-i18n.spec.ts`
+  - `frontend/angular/src/app/i18n-mvp-flow-smoke.spec.ts`
+  - `frontend/angular/src/app/features/host/host-shell.component.spec.ts`
+  - `frontend/angular/src/app/features/player/player-shell.component.spec.ts`

docs/ISSUE-175-I18N-SHARED-CONTRACT-ARTIFACT.md

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+# ISSUE-175 Artifact — shared i18n contract + MVP host/player flow
+
+## Delivered
+- Shared i18n contract is centralized in `shared/i18n/lobby.json` (da/en, default `en`, robust fallback).
+- Frontend host/player MVP copy is read via shared keys (no hardcoded Danish strings in Angular MVP flow components).
+- Backend error messages resolve from shared keyspace with locale-aware API payload (`error_code`, `error`, `locale`).
+- Audio-routing policy is explicit and shared: `frontend.capabilities.client_has_no_audio_output=true`.
+- Architecture + key naming documented in `docs/I18N_ARCHITECTURE.md`.
+
+## Smoke / e2e evidence
+### Backend locale + fallback
+Command:
+```bash
+. .venv/bin/activate && python manage.py test \
+  lobby.tests.LobbyFlowTests.test_join_error_localizes_to_danish_with_accept_language_header \
+  lobby.tests.LobbyFlowTests.test_join_error_falls_back_to_english_for_unsupported_locale \
+  lobby.tests.I18nResolverTests
+```
+Result:
+- PASS (7 tests)
+- Confirms `da` localization and fallback to `en` for unsupported locale.
+
+### Frontend host/player + language switch + audio policy
+Command:
+```bash
+cd frontend/angular
+npm test -- --run \
+  src/app/lobby-i18n.spec.ts \
+  src/app/i18n-mvp-flow-smoke.spec.ts \
+  src/app/features/host/host-shell.component.spec.ts \
+  src/app/features/player/player-shell.component.spec.ts
+```
+Result:
+- PASS (22 tests)
+- Includes smoke for host/player copy in **both en and da** and verifies primary-only audio policy flag (`clientHasNoAudioOutput=true`).

frontend/angular/src/app/i18n-mvp-flow-smoke.spec.ts

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+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { HostShellComponent } from './features/host/host-shell.component';
+import { PlayerShellComponent } from './features/player/player-shell.component';
+import { setPreferredLocale } from './lobby-i18n';
+
+describe('i18n MVP flow smoke (host/player + audio policy)', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+    vi.unstubAllGlobals();
+  });
+
+  it('resolves host/player copy in en and da from shared catalog', () => {
+    vi.stubGlobal('window', {
+      location: { hash: '', search: '' },
+      history: { state: null, replaceState: vi.fn() },
+      localStorage: { getItem: vi.fn().mockReturnValue('en'), setItem: vi.fn(), removeItem: vi.fn() },
+      sessionStorage: { getItem: vi.fn().mockReturnValue(null), setItem: vi.fn(), removeItem: vi.fn() },
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    });
+    vi.stubGlobal('navigator', { language: 'en-US', onLine: true });
+
+    const host = new HostShellComponent();
+    const player = new PlayerShellComponent();
+    host.ngOnInit();
+    player.ngOnInit();
+
+    expect(host.copy('host.start_round')).toBe('Start round');
+    expect(player.copy('player.submit_guess')).toBe('Submit guess');
+
+    setPreferredLocale('da');
+
+    expect(host.copy('host.start_round')).toBe('Start runde');
+    expect(player.copy('player.submit_guess')).toBe('Send gæt');
+
+    player.ngOnDestroy();
+    host.ngOnDestroy();
+  });
+
+  it('keeps audio routing policy primary-only (client has no audio output)', () => {
+    vi.stubGlobal('window', {
+      location: { hash: '', search: '' },
+      history: { state: null, replaceState: vi.fn() },
+      localStorage: { getItem: vi.fn().mockReturnValue('en'), setItem: vi.fn(), removeItem: vi.fn() },
+      sessionStorage: { getItem: vi.fn().mockReturnValue(null), setItem: vi.fn(), removeItem: vi.fn() },
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    });
+    vi.stubGlobal('navigator', { language: 'en-US', onLine: true });
+
+    const host = new HostShellComponent();
+    const player = new PlayerShellComponent();
+
+    expect(host.clientHasNoAudioOutput).toBe(true);
+    expect(player.clientHasNoAudioOutput).toBe(true);
+
+    player.ngOnDestroy();
+    host.ngOnDestroy();
+  });
+});