F-8 from the t-paliad-074 audit. Replaces silent `?? key` fallback with a
typed key surface so drift caught at compile/build time, not in prod.
- New `frontend/src/i18n-keys.ts` (generated): `I18nKey` literal union of
all 1288 keys in `i18n.ts`. Regenerated by `frontend/build.ts` on every
build; written only when content changes (no spurious diffs).
- `t(key: I18nKey)` is now strict — `t("fristn.detail.title")` fails
`tsc --noEmit`. New `tDyn(key: string)` is the explicit escape hatch
for runtime-composed keys (`tDyn(\`fristen.status.${x}\`)`); 27 dynamic
call sites migrated.
- Build-time scan in `build.ts` walks `src/**/*.{ts,tsx}` for literal
`data-i18n` / `data-i18n-placeholder` / `data-i18n-title` attributes
and aborts the build on any value not in the key set. Skips `${...}`
interpolations (can't resolve statically). Applied before bundling so
no artefact ships when an unknown literal is present.
Surfaced and fixed during migration:
- `data-i18n="fristen.save.modal.project"` (fristenrechner.ts:145) →
`fristen.save.modal.akte` — F-04-class bug, would render the raw key.
- `t("termine.field.project.none")` (appointments-new.ts:30) →
`termine.field.akte.none` — same class.
- `t("checklisten.instance.project.open")` (checklists-instance.ts:155)
→ `checklisten.instance.akte.open` — same class.
- 4 duplicate-key entries in `i18n.ts` removed (TS1117): `nav.termine`
and `akten.detail.tab.termine` each appeared twice in DE and twice in
EN with identical values.
Out of scope (per brief): the German-vs-English i18n-key namespace split
flagged as F-9, JSX intrinsic typing, and the `akten` → `projects`
half-rename in checklists-detail.ts. Those stay tsc-noisy until separate
tasks land.
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