Build container image in CI, bake in datasets, configure auth via env

- Dockerfile bakes the app and every CSV in data/ into an nginx image
- UI discovers data/*.csv via nginx JSON autoindex and offers a
  dropdown when there is more than one dataset
- docker-entrypoint.d/40-basic-auth.sh creates .htpasswd from
  BASIC_AUTH_USER / BASIC_AUTH_PASSWORD(_FILE) and refuses to start
  without credentials; replaces set-password.sh
- Gitea Actions workflow builds and pushes
  registry.traberph.de/public/interval_viz (latest, sha-*, semver)
- Escape resets the chart zoom

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

html/index.html

@@ -15,7 +15,8 @@
   <header>
     <h1 data-i18n="title">Heatpump compressor cycles</h1>
     <div class="header-right">
-      <span class="sub" id="source" data-i18n="loading">loading…</span>
+      <select id="dataset" hidden aria-label="Dataset"></select>
+      <span class="sub" id="source"></span>
       <div class="lang" role="group" aria-label="Language">
         <button type="button" data-lang="en">EN</button>
         <button type="button" data-lang="de">DE</button>