Add tzdata (closes #4)

Install tzdata from apt so the time zone database is present at run
time, and check a zone lookup in the smoke test.

Model: deepseek-v4-pro

build.sh

@@ -26,7 +26,8 @@ apt-get -y install --no-install-recommends \
     libbz2-dev libffi-dev libgdbm-dev liblzma-dev libncurses-dev \
     libreadline-dev libsqlite3-dev libssl-dev libxml2-dev libxmlsec1-dev \
     tk-dev uuid-dev zlib1g-dev \
-    shellcheck
+    shellcheck \
+    tzdata
 
 # --- Go (latest) -------------------------------------------------------------
 gover="$(curl -fsSL 'https://go.dev/VERSION?m=text' | head -n1)"
@@ -102,6 +103,7 @@ go version
 govulncheck -version
 golangci-lint version
 shellcheck --version
+test -e /usr/share/zoneinfo/America/New_York
 hugo version
 node --version
 npm --version