Document optional environment variables

[AnasAA98]

Description of Change

Documents four places where the README doesn't match the actual behavior
of the repo.

1. npm run build is required before npm test. The error-page
   tests assert that styles are inlined, which needs the compiled
   public/css/errors.css produced by npm run build:inline. CI runs
   npm ci → npm run build → npm test, but the README's Tests
   section only mentions npm test. On a fresh clone that means
   test/functional/pages.test.js fails with no explanation of why.

2. The Node version note was out of date. It said "Node v8 or
   higher." package.json sets engines.node to >=18.x, and the repo
   also pins v24.4.1 in .nvmrc, which the README never mentioned.
   Now points at .nvmrc instead of a hardcoded number so it can't
   drift again.

3. Eleven environment variables read by the server were
   undocumented. Added a Configuration table covering PORT,
   TRUST_PROXY, PATH_PREFIX, DRIVE_TIMEOUT_SECONDS,
   LIST_UPDATE_DELAY, EDIT_CACHE_DELAY, ALLOW_INLINE_CODE,
   REDIRECT_URL, TEST_EMAIL, GOOGLE_APPLICATION_CREDENTIALS, and
   LOG_LEVEL, with defaults read from the source.

4. Corrected the drive refresh interval. The App structure section
   said the tree is repopulated "on an interval (currently 60s)."
   server/list.js uses LIST_UPDATE_DELAY || 15, and the comment
   directly above it says 15s. Updated to match and to name the
   environment variable.

The new section was added to the doctoc TOC by hand. I didn't re-run
doctoc, because the current version also rewrites the "Using Docker"
entry (the badge image in that heading confuses it) and drops a blank
line, which would have added unrelated churn to this diff.

Related Issue

None. The LOG_LEVEL row notes that the value is currently ignored and
links #379, which already fixes it.

Motivation and Context

Reproduced on a clean clone at 0c63f81, Node v24.4.1 (matching
.nvmrc), macOS.

Following the README exactly (npm install --no-optional, then
npm test):

FAIL test/functional/pages.test.js
  ● Server responses › that return HTML › should render an inline
    <style> tag and no JS on error pages

    expected '<!DOCTYPE html>…' to match /<style type="text\/css">[^<]/i
    at test/functional/pages.test.js:137:31

Test Suites: 1 failed, 1 skipped, 12 passed, 13 of 14 total
Tests:       1 failed, 3 skipped, 114 passed, 118 total

After npm run build:

Test Suites: 1 skipped, 13 passed, 13 of 14 total
Tests:       3 skipped, 115 passed, 118 total

(The 1 skipped suite is the intentionally disabled
test/functional/playlists.test.js, unrelated to this change.)

Checklist

• Ran npm run lint and updated code style accordingly
• npm run test passes
• PR has a description and all contributors/stakeholder are noted/cc'ed
• tests are updated and/or added to cover new code — documentation only, no code changes
• relevant documentation is changed and/or added