Update docs and AI assistant for passkey biometric login

- HelpKnowledgeBase: passkey entry under USER PROFILE section with
  full how-it-works detail (setup, login flow, browser requirements,
  account-lock enforcement, per-device management)
- UserProfile help article: new Passkeys & Biometrics section between
  Two-Factor Auth and Appearance, with setup steps, login steps,
  browser compatibility note, and lost-device warning
- TOC nav link added to UserProfile article sidebar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Helpers/HelpKnowledgeBase.cs

@@ -969,9 +969,21 @@ public static class HelpKnowledgeBase
         - Upload a profile photo
         - Choose display theme (light/dark)
         - Manage two-factor authentication settings
+        - Register passkeys for biometric login (Face ID, fingerprint, Windows Hello)
 
         **Two-Factor Authentication:** [/TwoFactorSetup](/TwoFactorSetup) — Set up or manage 2FA for your account.
 
+        **Passkeys & Biometric Login:** [/Passkey/Manage](/Passkey/Manage) — Register your phone's Face ID, fingerprint, or Windows Hello so future logins don't require typing a password.
+
+        ### How passkeys work
+        - Log in once with your password as normal.
+        - After login, a prompt appears in the bottom-right corner offering to enable biometric login on that device.
+        - Click Enable and follow the device prompt (Face ID on iPhone, fingerprint/face on Android, Windows Hello on PC).
+        - Next time you log in, tap "Use Face ID / Biometric" (or the platform equivalent) on the login page — no password needed.
+        - Each device you enroll appears on the Passkeys & Biometrics page. You can remove individual devices at any time.
+        - The server always checks that your account is active and not locked before allowing passkey login — a disabled account cannot bypass this with a passkey.
+        - Passkeys require HTTPS and a compatible browser (Safari 16+, Chrome 108+, Edge 108+, or any modern Android browser).
+
         ---
 
         ## BILLING & SUBSCRIPTION