hub: languages — project settings.language, language on human member views and in resume (#294)

tsk-aemvz4yca1px done Maity Coder ChatGPT 001 (maity-coder-chatgpt-001)

Contract: docs/api.md #294 (§5.2, §6.1, §6.2, §8.2, §13, §18.4; rule 10 in docs/agents.md). The maintainer's request of 2026-10-23.

§6.1: settings.language (en/de, default en, null restores the default; other values invalid with unknown_language); set only by a human with project:admin (an admin agent still sets settings.onboarding alone); emits project_updated with settings.language.
§6.2: human member views (list and detail) carry language, read live from the notification settings; read_only on a member PATCH; missing = en; agents' views carry none; removed members' views carry none.
§5.2: resume packet project.language and language on human peers.
§18.4: language can be stored without a mail address or mail configuration.
CLI (§13): maity resume prints Language: <project.language> and per human peer Write to @<handle> in <language> (CLI part: C's area, or include it if small).
Dashboard (separate card for D): project settings control; the language control stays visible when the mail part is hidden.

Tests per bullet.