fix: correct sample data, keep CSVs out of the JS bundle, and pick a valid groupBy

[austenstone]

Three first-run bugs, found by actually running the app against real exports.

The demo's Enterprise Members page was empty

sample-data.ts mapped enterprise-members.csv to the dormant users export. The real members file was never loaded, so the Enterprise Members page had nothing to render. Fixed, and the dormant-users sample is now loaded under its own (correct) name — 7 samples instead of 6.

Enterprise Members now shows 164 licenses in the demo, where it was previously blank.

15 MB of CSV was being shipped as JavaScript

Samples were imported with ?raw, which inlines the file contents into a JS chunk. The build emitted a 15.4 MB JS chunk that the engine has to parse as source before anything renders.

Switching to ?url + fetch() makes Vite emit the files as static assets. Same bytes over the wire, but they're now a plain .csv the browser streams instead of a script it has to parse. Tests are unaffected — they read examples/ through readFileSync.

The first chart rendered empty for summarized reports

addReport hardcoded groupByColumn to 'username'. GitHub's summarized metered usage export has no username column (see #105), so uploading one landed you on a chart grouped by a column that doesn't exist — empty chart, no error, nothing to tell you why.

resolveGroupByColumn() samples the parsed rows for a populated value and falls back to the schema default. It samples with a stride rather than from the head, because username is legitimately blank on leading storage rows.

While in there: activeReportIndex was set from prev.reports.length instead of the post-filter count. When samples are auto-removed on a real import, that pointed past the end of the array. Currently masked, but wrong.

Verified

Uploaded a 20,000-row summarized export over the demo:

• 7 samples correctly auto-removed, leaving 1 report
• lands on groupBy=sku instead of the empty username
• renders $9,154.02 gross / 46.8K minutes across a populated SKU breakdown

260 tests pass, 0 lint errors, 0 type errors.

Stacked on #105. Review that one first.