Phase 3: Skills & Context — skill system, personal context, context layering

Backend:
- Skill model + migration (with FK on chats.skill_id)
- Personal + general skill CRUD services with access isolation
- Admin skill CRUD endpoints (POST/GET/PATCH/DELETE /admin/skills)
- User skill CRUD endpoints (POST/GET/PATCH/DELETE /skills/)
- Personal context GET/PUT at /users/me/context
- Extended context assembly: primary + personal context + skill prompt
- Chat creation/update now accepts skill_id with validation

Frontend:
- Skill selector dropdown in chat header (grouped: general + personal)
- Reusable skill editor form component
- Admin skills management page (/admin/skills)
- Personal skills page (/skills)
- Personal context editor page (/profile/context)
- Updated sidebar: Skills, My Context nav items + admin skills link
- English + Russian translations for all skill/context UI

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/public/locales/en/translation.json

@@ -36,7 +36,8 @@
     "admin": "Admin",
     "users": "Users",
     "context": "Context",
-    "skills": "Skills"
+    "skills": "Skills",
+    "personal_context": "My Context"
   },
   "dashboard": {
     "welcome": "Welcome, {{name}}",
@@ -64,6 +65,31 @@
     "version": "Version",
     "characters": "characters"
   },
+  "skills": {
+    "my_skills": "My Skills",
+    "general_skills": "General Skills",
+    "no_skills": "No skills yet.",
+    "no_personal_skills": "You haven't created any personal skills yet.",
+    "create_personal": "Create Personal Skill",
+    "edit_personal": "Edit Personal Skill",
+    "create_general": "Create General Skill",
+    "edit_general": "Edit General Skill",
+    "name": "Name",
+    "name_placeholder": "e.g. Cardiologist",
+    "description": "Description",
+    "description_placeholder": "Brief description of this specialist",
+    "system_prompt": "System Prompt",
+    "prompt_placeholder": "Instructions for the AI when using this skill...",
+    "icon": "Icon",
+    "no_skill": "No specialist",
+    "general": "General",
+    "personal": "Personal"
+  },
+  "personal_context": {
+    "title": "Personal Context",
+    "subtitle": "This context is added to all your AI conversations",
+    "placeholder": "Add personal information that the AI should know about you..."
+  },
   "common": {
     "loading": "Loading...",
     "error": "An error occurred",