Upgrade to DoenetML 0.7.21 and drop the parking standaloneUrl workaround

[dqnykamp]

Summary

Release 0.7.21 ships the dark-mode and memory/coordination work from the recent PRs (#42–#45), and gates windowed-mounting parking on version ≥ 0.7.21 (PARK_MIN_VERSION in the iframe wrapper). This upgrades the dependency and updates every DoenetML version reference to 0.7.21.

Changes

• Dependency: bump @doenet/doenetml-iframe to 0.7.21 (peer ^0.7.21 + dev 0.7.21) and refresh package-lock.json.
• Sample activity: update every document version in dev/testActivity.json to 0.7.21.
• Windowed cypress specs: the specs pinned their docs at 0.7.4 and passed a host-specified standaloneUrl purely so the wrapper would treat the viewers as parking-capable. With docs at 0.7.21, parking is gated on by version, so:
  • drop the standaloneUrl/cssUrl props from both mounts (bundle/CSS auto-resolve from the doc version),
  • remove the now-dead WINDOWED_STANDALONE_URL / WINDOWED_STANDALONE_CSS_URL helpers and their comment.
• Package version: bump @doenet/assignment-viewer to 0.1.0-alpha-16.

The darkMode and viewerUrls specs still pass the generic STANDALONE_URL for their own rendering assertions (not parking) and are left unchanged.

Notes

• Test fixtures in src/test/collectDoenetmlVersions.test.ts intentionally use mixed versions (0.7.4, 0.6.5, 0.7) to exercise mixed-version detection, so they are deliberately left as-is.
• Verified @doenet/doenetml-iframe@0.7.21 and @doenet/standalone@0.7.21 are published; npm run build, tsc --noEmit, and the vitest suite all pass.

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