M7: localization layer (#128)

UI strings come from per-language TOML catalogs compiled in (English text as the key and fallback, named placeholders, one/other plurals, context suffixes); command labels are translated at display time while ids, CLI, MCP and control output stay English; accessibility names follow. Preferences > Language (System / English / Español, live; the system language from the OS or the browser), pref.language for scripts. A key check test guards the catalogs. The Spanish catalog is a machine draft awaiting native review; docs/translating.md explains adding a language.

apps/figcraft-web/src/web.rs

@@ -86,4 +86,8 @@ impl Services for WebServices {
     fn reduce_motion(&self) -> bool {
         self.reduce_motion.as_ref().is_some_and(web_sys::MediaQueryList::matches)
     }
+    /// The browser's language (`navigator.language`).
+    fn system_language(&self) -> Option<String> {
+        web_sys::window().and_then(|w| w.navigator().language())
+    }
 }