refactor: restructure project to standard Go layout

- Reorganize code into cmd/bilidown and internal/ packages
- Rename client/ to web/ for frontend source
- Remove systray dependency for headless web service
- Embed static files into binary using go:embed
- Update import paths to use internal/ prefix
- Update .gitignore with common patterns

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/util/semaphore.go

@@ -0,0 +1,28 @@
+package util
+
+import "sync"
+
+type Semaphore struct {
+	ch chan struct{}
+	wg sync.WaitGroup
+}
+
+func NewSemaphore(concurrency int) *Semaphore {
+	return &Semaphore{
+		ch: make(chan struct{}, concurrency),
+	}
+}
+
+func (s *Semaphore) Acquire() {
+	s.ch <- struct{}{}
+	s.wg.Add(1)
+}
+
+func (s *Semaphore) Release() {
+	<-s.ch
+	s.wg.Done()
+}
+
+func (s *Semaphore) Wait() {
+	s.wg.Wait()
+}