Files previews: the cache is one database file, <data dir>/thumbs.db

bbolt instead of a tree of hash-named JPEGs: pure Go, so the binary stays static; one
file per machine; an entry carries where the preview came from (engine, codec, size,
when) beside the picture, and a count rides along so the tab can say what it holds
without walking anything. A miss never creates the file, so a machine nobody browses
with previews on never has one; a cache that cannot be opened is logged and previews
are still made and served, just not kept.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

go.mod

@@ -5,6 +5,7 @@ go 1.26.0
 require (
 	github.com/coder/websocket v1.8.15
 	github.com/creack/pty v1.1.24
+	go.etcd.io/bbolt v1.5.0
 	golang.org/x/crypto v0.57.0
 	golang.org/x/term v0.46.0
 )