feat!: upgrade to frontend-base 2.0

[arbrandes]

Description

Takes the @openedx/frontend-base peer dependency to ^2.0.0-alpha || 0.0.0-dev, part of openedx/frontend-base#299. The range stays generic rather than naming a specific alpha, since a template should not pin copies of itself to whichever prerelease happened to be current. Nothing else was required, as the issue predicted: the template imports nothing from react-intl, declares no TypeScript type members for the new semicolon rule to catch, and bundles no config for App.defaultConfig to hold. Two things came along anyway. The empty config on the test site's app entry goes, since after ADR 0017 config is the operator's field. And the lockfile takes @openedx/paragon to 23.23.0, the floor frontend-base 2.0 requires, without which npm ci fails the peer check.

The second commit is the part specific to this repository being a template. README-template-frontend-app.rst is what every new App Repository starts from, and its Configuration section spoke only of per-app config. It now explains how getAppConfig resolves bundled defaultConfig below commonAppConfig below config, and which field a shipped default belongs in.

Closes #1033

LLM usage notice

Built with assistance from Claude.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.27%. Comparing base (7cc13e5) to head (d6c75be).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1034   +/-   ##
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  Coverage   27.27%   27.27%           
=======================================
  Files           9        9           
  Lines          22       22           
  Branches        1        1           
=======================================
  Hits            6        6           
  Misses         16       16           

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