feat(inbox): Make signal cards cleaner (#1406)

## Problem

Signal cards in Code show raw `source_product`/`source_type` as gray badges, while Cloud has refined per-source rendering with specific metadata and consistent "Product · Type" headers. As we now focus on the Code UI and discard the Cloud experience for the time being, it's time to make Code properly snazzy.

## Changes

Adding `SignalCardHeader` to all cards: colored product dot + source line (e.g. "Error tracking · New issue") + "Weight" badge

Source-specific cards for error tracking (fingerprint display), GitHub (issue number, colored labels, "View on GitHub"), Zendesk (priority/status/tags, "Open" link), LLM analytics (model, provider, trace ID).

Author: Twixes