Fix GA tracking on create account

[knolleary]

Noticed whilst debugging HS form tracking the recently added GA tracking was causing an error to be thrown if GA wasn't enabled.

The gtag function needed to be explicitly added to window so that it would be visible to code inside vue.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 41.93%. Comparing base (ac9b70b) to head (f044727).

Files	Patch %	Lines
frontend/src/pages/account/Create.vue	0.00%	3 Missing ⚠️
forge/routes/ui/index.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3568   +/-   ##
=======================================
  Coverage   41.93%   41.93%           
=======================================
  Files         623      623           
  Lines       23996    23996           
  Branches     5847     5847           
=======================================
  Hits        10062    10062           
  Misses      13934    13934           

Flag	Coverage Δ
backend	78.35% <0.00%> (ø)
frontend	1.94% <0.00%> (ø)

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