Fix: Knowledge categories page shows hardcoded data for non-admin users #910

Problem

/knowledge/categories page appears to work but silently shows hardcoded fake category data for non-admin users instead of real data.

Root Cause (from debug-autobot investigation)

1. /api/knowledge_base/categories/main requires check_admin_permission dependency
2. Non-admin users get a 401/403 from the backend
3. Frontend silently catches the error and falls back to 3 hardcoded category stubs
4. No error is shown — users see fake data with no indication of failure

Fix Plan

Fix A: Remove admin requirement from /api/knowledge_base/categories/main

• The 3 main categories (autobot-documentation, system-knowledge, user-knowledge) are public metadata, not sensitive data
• Any authenticated user should be able to list the top-level categories in their own knowledge base

Fix B: Add proper frontend error handling

• Show a real error message (or 'requires admin' notice) instead of silently falling back to hardcoded data
• Add loading/error UI state to the categories load path in KnowledgeBrowser.vue and KnowledgeCategories.vue

Impact

High — all non-admin users see wrong data on the knowledge categories page with no indication anything is wrong