fix(www): allow committing build artifacts and simplify embed directive

[KCarretto]

This PR fixes a recurring build issue where go build fails due to missing or mismatched frontend build artifacts.

Changes:

1. tavern/internal/www/.gitignore: Added !/build to ensure the build directory is tracked by git. This allows the frontend artifacts to be committed, so go build works out-of-the-box without requiring npm run build in fresh checkouts.
2. tavern/internal/www/http.go: Changed the //go:embed directive from a list of glob patterns (build/*.html, build/*.png, etc.) to //go:embed build. This recursively embeds the entire build directory, which is simpler and more robust (e.g., it now includes site.webmanifest which was previously missed, and won't fail if .ico files are absent).

Verified by running go build ./tavern successfully.

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PR created automatically by Jules for task 15740604039805952117 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2658    ±0	2658    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1149	✅	2658	2658	0	0	0	0	38.0s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2658	0	0	5.5s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
TestDockerExecutor_Build_ContextCancellation	1	5.5s	5.5s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.1s	5.1s
TestInteractiveShell	1	5.0s	5.0s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: install::tests::test_install_execution	3	1.5s	4.5s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_error	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚