Rename twig template folders and files to snake_case

[loevgaard]

Fixes #115

The four templates move to snake_case paths per modern Symfony/Sylius convention (Form/theme → form/theme, Admin/Order/Show/Payment/* → admin/order/show/payment/*, with _operationsError also renamed to _operations_error), and all 8 reference sites follow (controller renders, the two prepended configs, and the four test expectations). CLAUDE.md's path reference updated too.

Free while 2.x is unreleased — 1.x shipped no templates, so no override paths exist in the wild and no upgrade note is needed, per the issue's BC analysis.

Verified: 73 tests, PHPStan (cold), ECS, lint:twig/lint:container green; the admin endpoint still responds after a cache clear. (Renames were done with two-step git mv so the case change is committed correctly despite the case-insensitive local filesystem.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.60%. Comparing base (af2574a) to head (2beed0b).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #116   +/-   ##
=========================================
  Coverage     79.60%   79.60%           
  Complexity      114      114           
=========================================
  Files            16       16           
  Lines           510      510           
=========================================
  Hits            406      406           
  Misses          104      104           

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