feat: add discovery-queue plugin

[Kava-4]

Summary

Adds Discovery Queue Auto as a Millennium plugin.

Repository: https://github.com/Kava-4/discovery-queue

Features:

• Automatically completes the Steam discovery queue via official store endpoints
• Claims free event stickers when available
• Steam toast notifications when queue trading cards or stickers are ready to collect
• Optional auto-run on the Discovery Queue page and auto-skip while browsing queue items
• Lightweight plugin (~13 KB installed, no backend)

Notes:

• Requires users to be logged into the Steam store inside the client
• Automates store queue actions during Steam sales/events only
• No API keys, telemetry, or external services
• Uses store.steampowered.com and ISaleItemRewardsService endpoints only

Test plan

• Enable plugin in Millennium and open Store → Discovery Queue
• Verify Auto Discover Queue button appears on /explore
• Verify queue completion and optional sticker claim during an active sale
• Verify reward notification toast when queue or sticker is available