refactor(api): type req<T>() and tighten the error layer (R7 S1)

req() returned Promise<any>, so every api.* return annotation was an
unchecked assertion and the 17 action endpoints leaked `any` to callers.
Make req generic with a single audited cast at the boundary; T flows from
each method's declared return type via contextual inference, so no call
site needs a type arg except where none existed:

- me(): type the /api/me probe ({ authenticated } + Partial<Me>)
- deepAll / syncSubscriptions: real backend-verified response shapes
- HttpError.detailData, localizeDetail(d), inner detailData: any -> unknown

Also teach localizeDetail the FastAPI 422 detail-ARRAY shape (carried in
from R6 S1b review): now that malformed requests return 422, surface the
offending field via a localized errors.validation key instead of falling
through to the generic toast.

No runtime change on normal flows (req is await-identical); the 422 branch
only fires on genuinely malformed input. Gate green: typecheck, eslint,
prettier, 68 vitest. Smoke-tested localdev: /api/me + feed 200, no console
errors.

frontend/src/i18n/locales/en/errors.json

@@ -1,6 +1,7 @@
 {
   "title": "Couldn't complete that",
   "generic": "The server couldn't carry out that action. Please try again.",
+  "validation": "Invalid value for “{{field}}”. Please check it and try again.",
   "server": "Server error",
   "ok": "OK",
   "offline": {

frontend/src/i18n/locales/hu/errors.json

@@ -1,6 +1,7 @@
 {
   "title": "Nem sikerült",
   "generic": "A szerver nem tudta végrehajtani a műveletet. Próbáld újra.",
+  "validation": "Érvénytelen érték: „{{field}}”. Ellenőrizd, majd próbáld újra.",
   "server": "Szerverhiba",
   "ok": "OK",
   "offline": {