feat: ship the interface in eight languages, with per-language plurals and mirroring

SPEC.md

@@ -73,6 +73,9 @@ The v1 client presents one reader screen:
 - Each library page carries a search box that narrows it by title or address, and each row carries its own actions: remove a bookmark, leave a site or check it now, forget a page. History lists one row per address, headed by the most recent visit, rather than repeating a page once per visit. Clearing history takes a second step on a page the reader can simply leave, since it cannot be undone.
 - A change is a link too: following `mews:do/<verb>` applies it and rerenders the page in place without adding a history entry. Only a link followed from one of the client's own pages is honored, so a fetched page cannot clear history or change a setting on the reader's behalf (spec 8.1). A form on one of these pages is routed the same way rather than sent to the network.
 - Our own pages are built from the same blocks as a page but are interface, not an article, so the renderer lays them out differently: wider before they stop growing, denser, rows divided by a line instead of bulleted, section headings quieter, a note under a row set smaller and dimmer, links carried by weight rather than an underline, and a one-field form on a single line.
+- The interface ships in English, German, Spanish, French, Hebrew, Japanese, Portuguese and Russian. It follows the device's language by default and the reader can choose another on the settings page, where each language is named in its own words so it can be found from an interface they cannot read. A message a language has not translated falls back to English rather than disappearing.
+- Hebrew mirrors the whole interface. Flutter's own widgets follow the same language, and our own pages take their direction from it; a fetched page keeps declaring its own, so an English interface still renders a Hebrew page right to left.
+- Plurals are separate messages rather than a rule inside one, and the choice is made in code from the language's own rules: Russian separates two to four from the rest, Hebrew has a form for two, French and Portuguese count zero as singular, and Japanese has one form. A language needing a form English lacks names it with the same id and that suffix.
 - Fonts are bundled, never fetched. The reader picks between two sans faces, Atkinson Hyperlegible and Luciole, and two serifs, Literata and Charis SIL; all four carry regular, italic, bold and bold italic, which the renderer needs for emphasis. The chosen face applies to the whole app, chrome included, so the address bar and the page below it are one face.
 - A character the chosen face has no glyph for is looked for in that face's own fallbacks before the platform is asked. Hebrew always comes from a bundled Hebrew face matched to the shape of the chosen one, so it never depends on what the device has installed. The serifs also cover Greek, Cyrillic and a wide Latin range and fall back to each other; the two sans faces are narrow and have no bundled sans to turn to, so for those the platform answers, and it answers a sans with a sans. Scripts nothing bundled covers, such as Arabic and the CJK languages, always come from the platform.
 - Adding a font is four TTFs in `app/assets/fonts/` with its licence beside them, one `fonts` entry in `app/pubspec.yaml`, one entry in `app/lib/application/fonts.dart`, and one label in `app/locales/en-US/main.ftl`. A family named in the registry but missing from pubspec would render in the platform's font without saying so, so `app/test/fonts_test.dart` checks that the two agree.