Move to Go 1.27, and fix initials for non-ASCII names

Initials() capped the loop with len(out) == 2, which counts bytes: a name
starting with Ä, Ö or Å filled the budget on its own and returned a single
letter. Count the initials taken instead.

go fix also wanted a strings.Builder here, but that allocates a string per
iteration to measure a two-character result. Took its SplitSeq suggestion
in lyrics.go, which drops an intermediate slice.

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.26-alpine AS build
+FROM golang:1.27-alpine AS build
 # CalVer, injected at build so no file needs bumping by hand: docker build --build-arg VERSION=…
 ARG VERSION=dev
 WORKDIR /src