Fix waveform seekbar staying stuck when playing radio after app restart

[systemfreund]

Fixes #1016

Problem

With "Use waveform seekbar" enabled: if Supersonic is closed while a normal track is loaded, then restarted (last track shown paused, waveform correctly rendered), starting an internet radio station afterwards leaves the previous track's waveform image displayed instead of clearing it — even though radio streams have no waveform. This only happens right after a restart; under normal operation switching to radio clears/switches the display correctly.

Root cause

Two related bugs, both around restoring a paused track on startup (LoadTrackPaused):

1. Stale flag causes waveform update to be skipped. onBeforeSongChange was overloaded for two purposes: (a) pre-generating a waveform image for the next-up track, and (b) detecting the special "just loaded paused at startup" case via engine.pendingLoadPaused. Because loadTrackPaused() also calls handleNextTrackUpdated() (to pre-cache the next queued track), and pendingLoadPaused is still true at that point, the "startup" branch fired a second time for the wrong item (the next-up track instead of the current one), leaving wasLoadTrackPaused stuck at true. That stale flag then made the next real OnSongChange event (e.g. starting an internet radio station) wrongly skip updating the waveform display, so the previous track's waveform stayed on screen.

   Fix: split the overloaded hook into two — onBeforeSongChange keeps its precache-only purpose, and a new onLoadTrackPaused hook fires exactly once, directly from loadTrackPaused, for the actual now-playing item. It can no longer be retriggered by unrelated precache calls.

2. Previously-masked off-by-one in cacheNextTracks(). With the double-invocation above removed, a second, previously-masked bug became visible: cacheNextTracks() only requests caching for play-queue indices > 0, excluding index 0. Since waveform generation piggybacks on the audio cache download (it waits for AudioCache.CacheFile() to have been called for the track), the currently-loaded-paused track's waveform would never start generating if that track happened to be at queue index 0 (e.g. the only track in the queue). Previously this was masked because bug (1) would coincidentally kick off caching/generation for the next track and display that instead. Fixed the off-by-one (idx > 0 → idx >= 0) so the current track is always included.

Testing

• go build ./..., go vet ./backend/..., go test ./backend/... all pass.
• Manually reasoned through both the single-track-queue and multi-track-queue startup scenarios to confirm the waveform is now generated/displayed for the correct (current) track exactly once, and correctly clears when switching to a radio station.