AI Builder voice input, onboarding improvements, and tracking updates

AI Builder shipped several updates: voice input is back and live for users to speak prompts instead of typing on the prompt page. The onboarding flow now features a cleaner mode selection screen for choosing between Manual and Agentic modes, and smart routing that automatically directs users to Agentic mode in the free trial flow when their prompt clearly fits that mode. Additionally, a comprehensive tracking overhaul added unified Amplitude events across the AI Builder flow in hPanel and builder-frontend, including new properties like building_method, prompt_length, method_switched, and entry_source to track user funnels from various sources.