fix(npm) :: install dependencies only once at root level

[81reap]

Motivation

• The browser tests carry their own package-lock.json, so a contributor installs npm dependencies twice and CI caches only one of the two lockfiles.
• Running the suite from the root leaves test-results/ and playwright-report/ behind, neither of them ignored.

Description

• Declare tests/end-to-end as a workspace of the root package, and delete its lockfile.
• Point the CI npm cache at the root lockfile, and run npm ci from the root in a job that otherwise defaults to tests/end-to-end.
• Install the dependencies before cd tests/end-to-end in CONTRIBUTING, since that is now where they live.
• Ignore test-results/ and playwright-report/ at the root.

Testing

• With tests/end-to-end/node_modules deleted, npm install from the root is enough for npx playwright test to run: the suite passes its 41 tests with no second install.
• npm test checks 41 files, reports nothing, and passes the 15 unit tests.
• no console errors on card page fails for the reason described in fix(biome.js) :: fix remaining lint issues #1366, on this branch and on the unmodified base alike.

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GitHub cannot base a pull request on a branch that lives in a fork, so all eight target main and each one carries the commits of those above it. Review and merge them in order:

1. fix(biome.js) :: fix remaining lint issues #1366 :: fix(biome.js) :: fix remaining lint issues
2. fix(modal) :: give modal component an accessible name #1367 :: fix(modal) :: give modal component an accessible name
3. fix(map) :: ignore map coordinates that are not a pair of numbers #1368 :: fix(map) :: ignore map coordinates that are not a pair of numbers
4. feat(chart) :: render column charts as bar charts #1369 :: feat(chart) :: render column charts as bar charts
5. fix(chart) :: align stacked series on their X values #1370 :: fix(chart) :: align stacked series on their X values
6. fix(chart) :: line series up on a category axis for every chart type #1371 :: fix(chart) :: line series up on a category axis for every chart type
7. fix(npm) :: install dependencies only once at root level #1372 :: fix(npm) :: install dependencies only once at root level ← this PR
8. feat(typescript) :: typecheck browser JavaScript in CI #1373 :: feat(typescript) :: typecheck browser JavaScript in CI