Server-render the GTM container snippet

[thostetler]

GoogleTagManager was rendered inside using next/script at the default
afterInteractive strategy, which is client-only in the pages router. The
container snippet was missing from server HTML and only appeared at the end of
body after hydration, which GTM's console flagged as "tag not placed correctly"
across / and /abs/* URLs.

• Inlines Google's canonical snippet as the first child of in _document
• Gates the snippet on NEXT_PUBLIC_GTM_ID being set, avoiding a gtm.js?id=undefined request
• Removes the client-side GoogleTagManager render from _app and the test provider
• Keeps sendGTMEvent, which only touches window.dataLayer

[github-actions]

Bundle size

Shared by all pages: 624.6 kB (-0.0 kB) ⚪

No route changed by more than 1 kB. ✅

_{First load = polyfills + shared _app chunks + route chunks, gzipped.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.9%. Comparing base (d6e8ab2) to head (7b589a1).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #935     +/-   ##
========================================
+ Coverage    67.9%   67.9%   +0.1%     
========================================
  Files         379     378      -1     
  Lines       44188   44081    -107     
  Branches     2380    2378      -2     
========================================
- Hits        29974   29930     -44     
+ Misses      14165   14103     -62     
+ Partials       49      48      -1     

Files with missing lines	Coverage Δ
src/test-utils.tsx	88.1% <100.0%> (-0.3%)	⬇️

... and 3 files with indirect coverage changes

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[thostetler]

Folded into #934 — the GTM placement fix and the user id hook are being tested and shipped together.