[8222] Add Google Tag Manager

[n-lark]

Description

See #8222
Test plan #8222 (comment)

Separate issue to add to helm chart is here: FlowFuse/helm#1008

Related Issue(s)

Resolves #8222

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template > Helm chart: expose Google Tag Manager config helm#1008
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production > Change: Add Google Tag Manager to Production https://github.com/FlowFuse/CloudProject/issues/1549
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.19%. Comparing base (4732870) to head (873aa85).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
forge/routes/ui/index.js	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8225      +/-   ##
==========================================
- Coverage   76.20%   76.19%   -0.01%     
==========================================
  Files         443      443              
  Lines       23728    23731       +3     
  Branches     6315     6316       +1     
==========================================
  Hits        18081    18081              
- Misses       5647     5650       +3     

Flag	Coverage Δ
backend	76.19% <0.00%> (-0.01%)	⬇️

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[cstns]

Marketing wants Google Tag Manager (container GTM-55L36797) on the free-trial signup page (/account/create).

We're injecting the gtm script globally, we could do it on a per page basis by loading/unloading on page enter/leave.

[n-lark]

Hey @cstns went global because of marketing. They can target pages themselves with a Page Path trigger, and I figured they'd want it on more pages down the line, so global was the flexible setup. We can scope it to /account/create if we'd rather, just note it'll never unmount after you hit that page without a refresh.