feat(player): crossfade, preload upgrades, and playback diagnostics

Crossfade (Preferences -> Experimental, Off by default):
- dual AVQueuePlayer design: the incoming track plays on a second player
  while the outgoing one fades out with an equal-power 20 Hz ramp over the
  actual time remaining, so the incoming track peaks exactly at the
  outgoing track's end.
- finishCrossfade promotes the incoming player to primary on ramp
  completion or on the outgoing item's DidPlayToEndTime, re-subscribing
  currentItem KVO and the periodic time observer and syncing now-playing,
  lyrics and scrobble state.
- pause, seek, skip, queue edits, clear-queue, and turning crossfade off
  cancel an in-flight fade; a failed incoming item cancels instead of
  fading into silence. The volume slider does not fight the ramp.
- Duration picker offers Off, 3, 5, 6, 8, 10 and 12 seconds; a positive
  duration enables crossfade and suppresses gapless preloading.

Gapless improvements:
- preloaded items now buffer with the system default instead of the 3 s
  stall-recovery cap; the cap is applied only once the item is current.
- when the stream cache completes after a remote preload was queued, swap
  it for the local file (remote -> local upgrade).
- refresh the EQ audio mix at bind only when the bypass state changed,
  instead of rebuilding a live MTAudioProcessingTap on every transition.
- transition diagnostics on Console.app (net.faultables.flo / player)
  gated behind the Debug setting: preload source, deadline arming,
  auto-advance, watchdog/fallback, and failure paths.

Tests: crossfade duration defaults to off, persists, and drives the
enabled state. 307 tests pass.

Merge pull request #153 from kepelet/release/2.5

Release/2.5

feat: polish tip jar sheet and persist per-tier tip counts

Show per-tier tip-count badges on the tip cards (top-right), recovered from
StoreKit transaction history so they survive an app reinstall on the same
Apple ID. Celebrate successful tips with a confetti burst and auto-dismiss.
Move the Support flo row below About flo in Preferences.

feat: create icons from Icon Composer

feat: show loading indicators on download tabs

some suggests to do preloading but just leave it as is for now

build(release): bump version to 2.1 (211)

build(release): bump version to 2.0 (205)

build(release): bump version to 1.7

build(release): bump version to 1.6.1

close #64
related with #59

build(release): bump build number