Drop .env handling, fix default dataset, update README

- Remove .env.example and its .gitignore entry; compose sets the
  dev credentials directly. Basic auth in the image is unchanged.
- Preselect the last dataset by name instead of by mtime, which is
  meaningless after a git checkout in CI
- README: Esc zoom reset, data update flow, data visibility note,
  runner image requirements, dev login for compose
- Remove duplicate data/history2.csv

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

html/app.js

@@ -348,11 +348,11 @@ async function init() {
   }
   state.datasets = files.map(f => f.name);
 
-  // last selection if still present, otherwise the most recently modified file
+  // last selection if still present, otherwise the last file by name
+  // (mtimes are useless here: git checkouts in CI give all files the same time)
   let saved = null;
   try { saved = localStorage.getItem(DATASET_KEY); } catch { /* storage unavailable */ }
-  const newest = files.reduce((a, b) => Date.parse(b.mtime) > Date.parse(a.mtime) ? b : a);
-  loadDataset(state.datasets.includes(saved) ? saved : newest.name);
+  loadDataset(state.datasets.includes(saved) ? saved : state.datasets[state.datasets.length - 1]);
 }
 
 init();