docs(i18n): add Angular host/player key-map bootstrap for MVP flow (#220)

docs/i18n-keymap.md

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+# i18n key-map bootstrap (Angular host/player MVP)
+
+Issue: #220  
+Scope: Lobby → Join → Start round → Round → Reveal → Scoreboard  
+Locales: `en`, `da`
+
+This document is the gameplay key-namespace map for Angular host/player MVP.
+It maps existing text keys only (no feature expansion) and stays aligned with `shared/i18n/lobby.json`.
+
+## Key families
+
+- `frontend.ui.host.*` — host-facing gameplay actions and status text.
+- `frontend.ui.player.*` — player-facing gameplay actions and status text.
+- `frontend.ui.common.*` — shared labels used across host/player views.
+- `frontend.errors.*` — user-facing error keys shown by frontend.
+- `backend.error_codes.*` / `contract.backend_to_frontend_error_keys.*` — backend code → frontend key bridge.
+
+## Gameplay flow key map
+
+| Flow step | Family | Key | en | da |
+|---|---|---|---|---|
+| Lobby | `frontend.ui.host` | `host.title` | Host gameplay flow | Vært gameplay-flow |
+| Lobby | `frontend.ui.player` | `player.title` | Player gameplay flow | Spiller gameplay-flow |
+| Lobby | `frontend.ui.common` | `common.session_code` | Session code | Sessionskode |
+| Lobby | `frontend.ui.player` | `player.nickname` | Nickname | Kaldenavn |
+| Join | `frontend.ui.player` | `player.join` | Join | Join |
+| Start round | `frontend.ui.host` | `host.start_round` | Start round | Start runde |
+| Round | `frontend.ui.host` | `host.show_question` | Show question | Vis spørgsmål |
+| Round | `frontend.ui.player` | `player.lie_label` | Lie | Løgn |
+| Round | `frontend.ui.player` | `player.submit_lie` | Submit lie | Send løgn |
+| Round | `frontend.ui.player` | `player.submit_guess` | Submit guess | Send gæt |
+| Reveal | `frontend.ui.host` | `host.mix_answers` | Mix answers → guess | Bland svar → gæt |
+| Reveal | `frontend.ui.host` | `host.calculate_scores` | Calculate scores → reveal | Udregn score → afslør |
+| Scoreboard | `frontend.ui.host` | `host.load_scoreboard` | Load scoreboard | Hent scoreboard |
+| Scoreboard | `frontend.ui.host` | `host.final_leaderboard` | Final leaderboard | Finale leaderboard |
+| Scoreboard | `frontend.ui.player` | `player.final_leaderboard` | Final leaderboard | Finale leaderboard |
+| Scoreboard | `frontend.ui.common` | `common.points_short` | pts | point |
+
+## Frontend error keys used in flow scope
+
+| Error family | Key | en | da |
+|---|---|---|---|
+| Join | `frontend.errors.session_code_required` | Session code is required. | Sessionskoden er påkrævet. |
+| Join | `frontend.errors.session_not_found` | Session code is invalid or the session no longer exists. | Sessionskoden er ugyldig, eller sessionen findes ikke længere. |
+| Join | `frontend.errors.nickname_invalid` | Nickname must be between 2 and 40 characters. | Kaldenavn skal være mellem 2 og 40 tegn. |
+| Join | `frontend.errors.nickname_taken` | Nickname is already taken. | Kaldenavnet er allerede taget. |
+| Join | `frontend.errors.join_failed` | Join failed. Check code or nickname and try again. | Kunne ikke joine. Tjek kode eller kaldenavn og prøv igen. |
+| Start round | `frontend.errors.start_round_failed` | Could not start round. Refresh the lobby and try again. | Kunne ikke starte runden. Opdater lobbyen og prøv igen. |
+| Any | `frontend.errors.unknown` | Action failed. Refresh status and try again. | Handlingen fejlede. Opdater status og prøv igen. |
+
+## Backend→frontend mapping for gameplay errors
+
+Mapped in `contract.backend_to_frontend_error_keys` (source: `shared/i18n/lobby.json`):
+
+- `session_code_required` → `session_code_required`
+- `nickname_invalid` → `nickname_invalid`
+- `session_not_found` → `session_not_found`
+- `session_not_joinable` → `join_failed`
+- `nickname_taken` → `nickname_taken`
+- `category_slug_required` → `start_round_failed`
+- `category_not_found` → `start_round_failed`
+- `round_start_invalid_phase` → `start_round_failed`
+- `round_already_configured` → `start_round_failed`
+- `host_only_action` → `start_round_failed`
+
+## Notes
+
+- This is a bootstrap key-map doc for MVP mergeability.
+- The key/value source of truth remains `shared/i18n/lobby.json`.