feat: merge Context page into Glossaries — single page for glossary + system prompt + presets

- Add system prompt textarea and professional presets (HVAC, IT, Legal, etc.) to Glossaries page
- Remove Context from sidebar navigation (constants.ts)
- Make GlossarySelector always visible for Pro users (not just LLM mode)
- Send system prompt from Zustand store to backend via custom_prompt
- Add 24 new i18n keys across all 13 locales for glossaries page

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/app/dashboard/translate/useTranslationSubmit.ts

@@ -147,6 +147,11 @@ export function useTranslationSubmit(): UseTranslationSubmitReturn {
       if (config.glossaryId) {
         formData.append('glossary_id', config.glossaryId);
       }
+      // System prompt from Context page (Pro only)
+      const { settings } = await import('@/lib/store').then(m => m.useTranslationStore.getState());
+      if (settings.systemPrompt?.trim()) {
+        formData.append('custom_prompt', settings.systemPrompt.trim());
+      }
 
       const token = localStorage.getItem('token');
       const headers: Record<string, string> = {};