feat: add global toggle to disable all AI features

[datlechin]

Summary

Closes #494

• Adds an "Enable AI Features" toggle in Settings > AI that completely disables all AI functionality when turned off
• When disabled: AI Chat tab hidden, context menu AI items removed, "Ask AI to Fix" button removed from error alerts, inline suggestions suppressed, Ollama auto-detection skipped, per-connection AI policy section hidden
• Defaults to enabled for existing users (backward-compatible)

Test plan

• Toggle off in Settings > AI → all subsections collapse
• Right sidebar shows only Details tab (no AI Chat)
• Editor right-click with selection → no "Explain/Optimize with AI" items
• Trigger a query error → no "Ask AI to Fix" button, just OK
• Connection form → no AI policy section
• Toggle back on → everything reappears
• Fresh install (no existing settings) → AI enabled by default