Document the nine languages, and what Beta means on them

The translations shipped today and the docs still said "no languages but
English". They also need to say the harder thing, which is that eight of
the nine have never been read by anybody who speaks them.

- README gains the language list, with the Beta caveat in the same line
  rather than a footnote.
- FEATURES.md gains an Interface language section: the list, why it is a
  separate setting from the date locale, and the two design properties
  that follow — a missing entry renders English, and plurals are asked of
  Intl.PluralRules rather than assumed, which is why Russian carries
  three forms and Japanese one.
- ROADMAP.md no longer lists translations as "not yet". What replaces it
  is the half that is genuinely not done: a translation anybody has
  checked. RTL is split out as its own entry, because holding Arabic,
  Hebrew and Persian back is a layout decision and not a queue position.
- KNOWN-ISSUES.md gains two entries. The unread catalogues, which is the
  one thing on that page that cannot be closed by testing. And the
  coverage number that read 100% while two hundred strings rendered
  English in every language — recorded as a general lesson rather than an
  i18n one, since a coverage number measures what it can see and the rest
  is exactly what nobody is checking.

README.md

@@ -51,6 +51,7 @@ More, including the mobile layout, on [ihasmail.org](https://ihasmail.org/#scree
 - **Files** — JMAP FileNode: browse, upload, download, rename, move, delete
 - **Settings that follow the account**, not the browser — kept in a `settings.json` in the account's own JMAP Files, so ihasmail itself stays stateless
 - **Runs read-only** — one optional write path, and with it switched off the container needs no volume and no writable root. `IMMUTABLE=1` is checked at startup rather than trusted, so a half-applied switch refuses to boot instead of failing quietly. See [Running immutably](#running-immutably)
+- **Nine interface languages** — English plus German, Spanish, French, Dutch, Portuguese (Brazil), Russian, Ukrainian, Simplified Chinese and Japanese, separate from the date-and-time locale. All but English are marked **Beta**: they were made by AI and no native speaker has read them yet, which Settings says plainly, with a link for reporting anything wrong
 - **On a phone** — swipe a message to archive or delete it (either direction, your choice), hold one to select it, hold a folder for its menu, pull the list to refresh, swipe back from a conversation
 - **Platform** — installable PWA, Web Push with ihasmail closed, `mailto:` handler, no credentials in the browser, strict CSP, SSRF-safe image proxy