Overhaul Proxy events

[aspage2]

Bug Fix: Runaway Goroutines

The startMPDIdler function has a bug which causes the idler goroutine to enter an infinite loop if its connection to MPD is broken. The issue is an incorrectly-used scanner; in the case that MPD closes the connection, sc.Scan() returns false but doesn't error. The Idler only exits on error, so it enters an infinite loop of calling sc.Scan() on a closed socket.

Fixes

• Switch to a bufio.Reader for error-handling at the time that the socket read occurs, as well as direct handling of io.EOF.
• Add error-handling code where there was none previously, returning from the idler if any socket error occurs
• Wrap the original idler logic in another function that manages dropped connections and re-attempts to connect to MPD
• Include a Sleep call to throttle the attempts to reconnect.

New Event Pub/Sub System

Create a new Topic type which supports single-topic publish/subscribe between goroutines.
Receivers can subscribe and unsubscribe from the topic, and publishers can create events on
the topic with Publish. Topic is thread-safe.

Refactored the /go/events endpoint to utilize a pub/sub topic to receive updates from the server.
Each SSE client subscribes to the topic, and the new "idler" goroutine publishes changes on the other end.

Refactor: Single idler, multiple listeners

Currently, each SSE client uses its own connection to the MPD server when using the "idle" command.
This PR refactors the event system using a pub/sub mechanism:

• Only one goroutine maintains an "idle" connection with the MPD socket.
• The goroutine receives server updates and publishes them on a channel-based pub/sub topic
• Any open SSE clients will subscribe to the event topic and receive any published events.

Frontend Changes

• Updated the event source object to handle the refactored event system
• New events for mpd connecting/disconnecting
• New connection status bubbles on the Settings page