refactor(src/lib): 1a - remove dead code

[sukhada]

Summary

Phase 1A of the modernization effort scoped specifically to the src/lib directory. Dead code identified via ts-prune + manual grep verification across all production code (excluding tests).

Deletions (-214 lines):

• dst-helper.js + its test — unused outside its own test; hand-rolls DST detection that luxon does natively
• js-types.ts — misleading types (TruthyString includes falsy values); inlined into the 2 consuming files with better names
• __mocks__/ directory — redundant; jest.mock() auto-mocks without needing manual mock files
• getInteractionTree — never imported outside barrel
• getDisplayPhoneNumber — never imported outside barrel
• getCommonZipRanges — never imported outside barrel
• timezonecomplete dependency — only consumer was dst-helper.js

Unexported (internal-only):

• getHighestRole — only used by hasRole() within permissions.ts
• downloadFromUrl — only used by withTempDownload() within utils.ts

Verification method: ts-prune --project tsconfig.json flagged candidates. Each was then verified with grep across the full codebase (excluding test/spec files and barrel re-exports) to confirm zero production consumers.

Test plan

• All 25 existing src/lib/ tests pass
• Lint and prettier clean
• CI passes

🤖 Generated with Claude Code