Add Russian (ru_RU) locale and stop silently falling back to English dates for unknown locales

Summary Russian is not among the shipped interface locales, and the client's date handling falls back to English for any language code it does not know. We run a self-hosted Outline (official image outlinewiki/outline, version 1.10.1, shared/i18n/locales contains 28 locales, ru_RU is not one of them) and translated the interface for our own use. Doing that surfaced two problems worth fixing upstream, and we would like to contribute the result. 1. Missing ru_RU locale Demand is clearly there: the community project flameshikari/outline-ru (133 stars, last update 2026-09-10, tracking version 1.10.1) ships Russian by forking and patching Outline. We have a complete translation instead: 1906 strings, keyed exactly like en_US, placeholders and markup preserved. Against current main (1920 keys) our file covers 1906 — the fourteen strings added since 1.10.1 are not translated yet, and that is precisely why we would rather regenerate against main before opening a pull request. 2. Date…