Initial lookbook implementation

Pinterest-style visual bookmarking app with:
- URL metadata extraction (OG/Twitter meta, oEmbed fallback)
- Image caching in Postgres with 480px thumbnails
- Multi-tag filtering with Ctrl/Cmd for OR mode
- Fuzzy tag suggestions and inline tag editing
- Browser console auth() with first-use password setup
- Brutalist UI with Commit Mono font and Pico CSS
- Light/dark mode via browser preference

vendor/github.com/jackc/pgx/v5/pgconn/ctxwatch/context_watcher.go

@@ -0,0 +1,80 @@
+package ctxwatch
+
+import (
+	"context"
+	"sync"
+)
+
+// ContextWatcher watches a context and performs an action when the context is canceled. It can watch one context at a
+// time.
+type ContextWatcher struct {
+	handler     Handler
+	unwatchChan chan struct{}
+
+	lock              sync.Mutex
+	watchInProgress   bool
+	onCancelWasCalled bool
+}
+
+// NewContextWatcher returns a ContextWatcher. onCancel will be called when a watched context is canceled.
+// OnUnwatchAfterCancel will be called when Unwatch is called and the watched context had already been canceled and
+// onCancel called.
+func NewContextWatcher(handler Handler) *ContextWatcher {
+	cw := &ContextWatcher{
+		handler:     handler,
+		unwatchChan: make(chan struct{}),
+	}
+
+	return cw
+}
+
+// Watch starts watching ctx. If ctx is canceled then the onCancel function passed to NewContextWatcher will be called.
+func (cw *ContextWatcher) Watch(ctx context.Context) {
+	cw.lock.Lock()
+	defer cw.lock.Unlock()
+
+	if cw.watchInProgress {
+		panic("Watch already in progress")
+	}
+
+	cw.onCancelWasCalled = false
+
+	if ctx.Done() != nil {
+		cw.watchInProgress = true
+		go func() {
+			select {
+			case <-ctx.Done():
+				cw.handler.HandleCancel(ctx)
+				cw.onCancelWasCalled = true
+				<-cw.unwatchChan
+			case <-cw.unwatchChan:
+			}
+		}()
+	} else {
+		cw.watchInProgress = false
+	}
+}
+
+// Unwatch stops watching the previously watched context. If the onCancel function passed to NewContextWatcher was
+// called then onUnwatchAfterCancel will also be called.
+func (cw *ContextWatcher) Unwatch() {
+	cw.lock.Lock()
+	defer cw.lock.Unlock()
+
+	if cw.watchInProgress {
+		cw.unwatchChan <- struct{}{}
+		if cw.onCancelWasCalled {
+			cw.handler.HandleUnwatchAfterCancel()
+		}
+		cw.watchInProgress = false
+	}
+}
+
+type Handler interface {
+	// HandleCancel is called when the context that a ContextWatcher is currently watching is canceled. canceledCtx is the
+	// context that was canceled.
+	HandleCancel(canceledCtx context.Context)
+
+	// HandleUnwatchAfterCancel is called when a ContextWatcher that called HandleCancel on this Handler is unwatched.
+	HandleUnwatchAfterCancel()
+}