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/sethvargo/go-retry/rand.go

@@ -0,0 +1,54 @@
+package retry
+
+import (
+	"math/rand"
+	"sync"
+)
+
+type lockedSource struct {
+	src *rand.Rand
+	mu  sync.Mutex
+}
+
+var _ rand.Source64 = (*lockedSource)(nil)
+
+func newLockedRandom(seed int64) *lockedSource {
+	return &lockedSource{src: rand.New(rand.NewSource(seed))}
+}
+
+// Int63 mimics math/rand.(*Rand).Int63 with mutex locked.
+func (r *lockedSource) Int63() int64 {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.src.Int63()
+}
+
+// Seed mimics math/rand.(*Rand).Seed with mutex locked.
+func (r *lockedSource) Seed(seed int64) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.src.Seed(seed)
+}
+
+// Uint64 mimics math/rand.(*Rand).Uint64 with mutex locked.
+func (r *lockedSource) Uint64() uint64 {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.src.Uint64()
+}
+
+// Int63n mimics math/rand.(*Rand).Int63n with mutex locked.
+func (r *lockedSource) Int63n(n int64) int64 {
+	if n <= 0 {
+		panic("invalid argument to Int63n")
+	}
+	if n&(n-1) == 0 { // n is power of two, can mask
+		return r.Int63() & (n - 1)
+	}
+	max := int64((1 << 63) - 1 - (1<<63)%uint64(n))
+	v := r.Int63()
+	for v > max {
+		v = r.Int63()
+	}
+	return v % n
+}