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/page.tsx

@@ -402,7 +402,7 @@ export default function TranslatePage() {
                       isPro={config.isPro}
                     />
 
-                    {config.isPro && config.mode === 'llm' && (
+                    {config.isPro && (
                       <GlossarySelector
                         glossaryId={config.glossaryId}
                         onChange={config.setGlossaryId}