feat: resolve #25 - improve downloader UI accuracy and ergonomics

[KenjiBaheux]

Overview

This PR addresses the user confusion regarding the 22GB storage requirement (Ref: Issue #25) and proposes improvement to the model initialization experience. It shifts the messaging from "22GB download" to a more accurate "system health requirement" while providing real-time feedback on which requirements are met.

Changes

UI & UX Enhancements

• Dynamic System Checks: Replaced static text with a live "System Check" list. It now detects if the Chrome flag is enabled and uses the Storage Manager API to check for available disk space. That said, the availability() API should already take this into consideration.
• Storage Clarification: Explicitly decoupled the 22GB system margin requirement from the actual ~2GB model download to prevent user drop-off.
• Indeterminate Progress States: Updated the progress bar to pulse (indeterminate state) during "Checking" and "Extracting" phases. This provides immediate visual feedback even when the download percentage is at 0%. This is a bit hard to test but I hope I got it right.
• Streamlined Requirements: Reordered compatibility items to lead with "low-friction" requirements (pre-requisites), moving the technical flag toggle to the end of the list.
• Integrated Copy Helper: Added a "hover-to-reveal" copy icon for the chrome://flags link to reduce manual effort for the user.

Related Issues

Fixes #25