feat: add account management, email verification, AI fixes, and user guides

- Profile settings, account transfer, delete/leave account flows
- Email verification with JWT tokens and Resend integration
- AI assistant/copilot fixes: markdown rendering, shared RAG helpers,
  token tracking, input refocus, model_validate usage
- User guides hub + detail pages with 13 topic guides
- Sidebar and top bar navigation for guides

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

backend/app/schemas/account.py

@@ -20,6 +20,11 @@ class AccountUpdate(BaseModel):
     name: Optional[str] = Field(None, min_length=1, max_length=255)
 
 
+class TransferOwnershipRequest(BaseModel):
+    current_password: str
+    target_user_id: UUID
+
+
 class AccountInviteCreate(BaseModel):
     email: str = Field(..., max_length=255)
     role: str = Field("engineer", pattern="^(engineer|viewer)$")