chore: prepare 2.0.0-beta.16

[skjnldsv]

Version bump and changelog for 2.0.0-beta.16: the video sizing and capped preloading (#114), and the viewer no longer remounting the file shown on an update of the same version (#119). #115, #117 and #118 were tests and CI only.

Text's port (nextcloud/text#9235) waits on #119: without it the editor is rebuilt on each of its own saves, which its end-to-end tests catch.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (d5be4b0) to head (e85e0a7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #120   +/-   ##
=======================================
  Coverage   91.07%   91.07%           
=======================================
  Files          41       41           
  Lines        3430     3430           
  Branches      792      792           
=======================================
  Hits         3124     3124           
  Misses        288      288           
  Partials       18       18           

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