fix: prevent webpack warning for optional date-fns-tz dependency

[martijnrusschen]

Summary

• Use a variable for the module name in the dynamic require to prevent webpack from statically analyzing it
• This eliminates the "Module not found: Error: Can't resolve 'date-fns-tz'" warning when the optional peer dependency is not installed

Root Cause

The require("date-fns-tz") statement used a string literal, which webpack's static analysis detected and flagged as a missing module—even though it was wrapped in a try-catch and date-fns-tz is listed as optional in peerDependenciesMeta.

Fix

Changed from:

dateFnsTz = require("date-fns-tz") as DateFnsTz;

To:

const dateFnsTzModuleName = "date-fns-tz";
dateFnsTz = require(dateFnsTzModuleName) as DateFnsTz;

This prevents webpack from statically analyzing the require and showing warnings.

Fixes #6154

Test plan

• All existing tests pass (1445 tests)
• Verified the built output uses the variable pattern
• Linter passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.25%. Comparing base (403732b) to head (78b9dfc).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6155      +/-   ##
==========================================
+ Coverage   99.18%   99.25%   +0.07%     
==========================================
  Files          30       30              
  Lines        3782     3783       +1     
  Branches     1644     1647       +3     
==========================================
+ Hits         3751     3755       +4     
+ Misses         29       27       -2     
+ Partials        2        1       -1     

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