Add full shell, replaceable Omics adapter and bilingual interaction

Implement approved specification #32 and tickets #33-#37. Keep host authentication server-verified and pin session interaction language. Compile scoped base selectors for browser compatibility and retain full gutters during CSS pruning.

backend/src/tht/interaction-language.ts

@@ -0,0 +1,10 @@
+/** Validate/canonicalize the tag; available translation catalogs belong to the UI. */
+export function interactionLanguage(value: unknown): string | undefined {
+  if (typeof value !== "string") return undefined;
+  try {
+    const [canonical] = Intl.getCanonicalLocales(value);
+    return canonical;
+  } catch {
+    return undefined;
+  }
+}