feat: add source/target language selectors to edit glossary dialog

Users can now change the language pair when editing a glossary:
- EditGlossaryDialog has source/target language dropdowns
- Default target_language changed from 'en' to 'multi' in create dialog
- onSave passes source_language and target_language to the backend
- Backend PATCH endpoint already supports updating these fields

Also fixes:
- CreateGlossaryDialog defaults to 'multi' instead of 'en'
- SUPPORTED_LANGUAGES now includes 'multi' option for target

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app/dashboard/glossaries/page.tsx

@@ -185,7 +185,7 @@ export default function GlossariesPage() {
     }
   };
 
-  const handleSaveGlossary = async (id: string, data: { name: string; terms: GlossaryTermInput[] }) => {
+  const handleSaveGlossary = async (id: string, data: { name: string; source_language: string; target_language: string; terms: GlossaryTermInput[] }) => {
     try {
       await updateGlossary(id, data);
       setEditDialogOpen(false);