fix: reset context metadata when loading a new context

[plisker]

What

Reset PlayerState.ContextMetadata when loading a new context in loadContext, instead of merging the new context's metadata into the map left over from the previous one.

Why

When switching from one context to another (e.g. starting an album while a different album is playing), every field of the new context — SessionId, ContextUri, ContextUrl, Restrictions, tracks — is replaced, but ContextMetadata was only merged. Keys the previous context set that the new one omits therefore survived. Most visibly, context_description lingered, so Spotify's queue kept showing "Next from: <previous album>" while the correct track played.

The transfer path in daemon/player.go already resets the map before copying; this brings loadContext in line. The same-URI update_context path still merges, which is correct there.

Test plan

• go build -v -o /dev/null ./cmd/daemon
• gofmt -l . clean
• On-device: play album A, start a track from album B, confirm the queue header reads album B.

Closes #330