chore: upgrade templ v0.3.1001 → v0.3.1020, pin in Dockerfile

Newer templ generates ResolveAttributeValue calls that don't exist in
v0.3.1001 runtime, causing Docker build failures ("undefined:
templ.ResolveAttributeValue"). Pin templ CLI version in Dockerfile to
match go.mod instead of using @latest.

Also updated local templ CLI to v0.3.1020 to match.

go.mod

@@ -4,7 +4,7 @@ go 1.26.0
 
 require (
 	github.com/ArcadiaLin/go-epub v0.1.1
-	github.com/a-h/templ v0.3.1001
+	github.com/a-h/templ v0.3.1020
 	github.com/andybalholm/brotli v1.2.1
 	github.com/bodgit/plumbing v1.3.0
 	github.com/bodgit/windows v1.0.1