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/constants.ts

@@ -10,7 +10,7 @@ export interface NavItem {
 export const baseNavItems: NavItem[] = [
   { labelKey: 'dashboard.nav.translate', href: '/dashboard/translate', icon: FileText },
   { labelKey: 'dashboard.nav.profile', href: '/dashboard/profile', icon: User },
-  { labelKey: 'dashboard.nav.context', href: '/dashboard/context', icon: Globe, proOnly: true },
+  { labelKey: 'dashboard.nav.glossaries', href: '/dashboard/glossaries', icon: BookText, proOnly: true },
   { labelKey: 'dashboard.nav.apiKeys', href: '/dashboard/api-keys', icon: Key, proOnly: true },
 ];