Add build version indicator to sidebar menu

Display commit date and short hash (e.g., "2026-01-08 fb59ef7") below
the ANIMALTRACK title in the sidebar. In development, reads from git
directly; in Docker, reads from BUILD_DATE/BUILD_COMMIT env vars
injected at build time from the Nix flake.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker.nix

@@ -1,4 +1,4 @@
-{ pkgs, pythonEnv, python }:
+{ pkgs, pythonEnv, python, buildDate ? "unknown", buildCommit ? "unknown" }:
 
 let
   # Build animaltrack as a package
@@ -59,6 +59,8 @@ pkgs.dockerTools.buildImage {
       "PATH=${pkgs.lib.makeBinPath [ pkgs.busybox pkgs.bash pkgs.sqlite pythonEnv animaltrack ]}"
       "PYTHONPATH=${pythonEnv}/${pythonEnv.sitePackages}:${animaltrack}/${pythonEnv.sitePackages}"
       "PYTHONUNBUFFERED=1"
+      "BUILD_DATE=${buildDate}"
+      "BUILD_COMMIT=${buildCommit}"
     ];
     ExposedPorts = {
       "5000/tcp" = {};