i18n: add Swedish (sv)

Adds a complete Swedish catalog (2,961 messages) and registers sv in the
locale list, catalogs, language picker and package boundaries. Browsers set
to Swedish get it at once; ?lang=sv or the language picker choose it
elsewhere. Numbers and dates come out Swedish from Intl.

- docs/translations/sv.md: style (imperative without a pronoun, closed
  compounds, decimal comma, space before units and %) and the glossary.
- Entries that stay identical to English on purpose (codes such as HDG and
  FOV, product, place and source names) are listed under sv in
  same-as-english.json.
- The picker-order test places Svenska after Português.
- docs/CURRENT-STATE.md and docs/TRANSLATORS.md list the new language.

The translation comes from God's Eye View Sverige, a Swedish fork, and was
brought up to date with main (wildlife sightings, event age filter, traffic
styles and the rest).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/CURRENT-STATE.md

@@ -4285,7 +4285,7 @@ test:track` 43 tracking invariants · headless QA harnesses under
 
 ### Interface localization (October 2026)
 
-Shipped locales: **en** (source and fallback), **ar**, **de**, **es**, **fr**, **it**, **ja**, **pt** (Brazilian Portuguese), **ru**, **uk**, **zh-CN** and **zh-Hant**. The code lives in
+Shipped locales: **en** (source and fallback), **ar**, **de**, **es**, **fr**, **it**, **ja**, **pt** (Brazilian Portuguese), **ru**, **sv**, **uk**, **zh-CN** and **zh-Hant**. The code lives in
 `src/i18n/`. How to mark UI text: [`docs/I18N.md`](./I18N.md); translator workflow:
 [`docs/TRANSLATORS.md`](./TRANSLATORS.md).