Fix sequential script loading in webtrees.load()

[magicsunday]

Summary

• Fix race condition in webtrees.load() where inline scripts execute before external scripts have finished loading
• Replace forEach with a Promise chain that waits for each external script's onload event before appending the next script

Problem

The webtrees.load() function (which replaced jQuery's .load()) dynamically injects <script> elements via appendChild. External scripts (with src) load asynchronously by default, but the forEach loop immediately appends the next script without waiting. This causes inline scripts that depend on a preceding external library to fail with errors like:

Uncaught TypeError: WebtreesFanChart.FanChart is not a constructor

This affects any custom module that uses the common pattern of pushing an external script tag followed by an inline initialization script via View::push('javascript').

Fix

Scripts are now loaded sequentially using a Promise chain. External scripts wait for their onload event before the next script in the chain is appended to the DOM.

Test plan

• Custom modules using View::push('javascript') with separate external and inline script blocks load correctly
• Tested with magicsunday/webtrees-fan-chart, webtrees-pedigree-chart, and webtrees-descendants-chart

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.37%. Comparing base (c338276) to head (ac31a62).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #5359   +/-   ##
=========================================
  Coverage     35.37%   35.37%           
  Complexity    11196    11196           
=========================================
  Files          1166     1166           
  Lines         48031    48031           
=========================================
  Hits          16990    16990           
  Misses        31041    31041           

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[magicsunday]

For reference, this regression was introduced in commit a3ca8e1 ("Convert jQuery code to VanillaJS"). The previous jQuery .load() implementation executed scripts sequentially, while the vanilla JS replacement uses forEach + appendChild which starts external script downloads without waiting for onload before appending the next script.

This affects all custom modules that use the common two-block pattern (View::push('javascript') with an external script followed by inline initialization).

[magicsunday]

Our modules no longer trigger this race condition — we switched to dynamic import().then() inside a single <script type="module">.

Keeping this PR open since the fix in webtrees.load() would still benefit other custom modules that use external script tags via View::push('javascript'). Happy to rebase if needed.