refactor: rewrite get_query_strings with URLSearchParams

[jacaudi]

Summary

• Replace hand-rolled get_query_strings parser (broken closure, dead return_as_array branch) with a 9-line URLSearchParams-based implementation preserving all coercion semantics (empty/null/true/false/integer).
• Add 11 table-driven unit tests covering every coercion branch, including the ?flag= edge case the old code got wrong.
• Drop the dead false argument from the sole caller in init.js.

Test plan

• docker run --rm -v $(pwd):/work -w /work node:22-alpine sh -c "node scripts/build.js && node --test app/tests/test-*.js" — 177/177 passing
• No return_as_array references remain in src/
• All callers use zero-arg get_query_strings()
• ?flag= now correctly returns { flag: true } (previously { flag: '' })

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