fix: repair failing utility tests across 5 modules

[stooit]

Summary

Fixes all 16 failing tests in the utility library. The full suite now passes: 60 pass, 0 fail. No tests were modified and no dependencies were added.

Fixes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0 instead of returning Infinity.
• src/string-utils.ts — wordCount splits on /\s+/ to collapse consecutive whitespace; truncate implemented (returns unchanged within limit, truncates at word boundary, ellipsis counts toward maxLength, handles strings shorter than the ellipsis).
• src/task-manager.ts — implemented remove (returns false for unknown id), update (title/priority, returns false for unknown id), and sortBy (priority high→medium→low; createdAt oldest first).
• src/date-utils.ts — fixed off-by-one in formatRelative: day count now rounds correctly so exactly 36 hours reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts URLs with a port (e.g. http://localhost:3000).

Verification

bun test → 60 pass, 0 fail, 70 expect() calls.

Assumptions / notes

• date-utils.formatRelative: the fix uses Math.round on the day count. This satisfies the test and shifts the hours→days display boundary. No test covers the 12-23h range; left minimal per the "fix only what the tests require" constraint.
• string-utils.truncate: when maxLength <= ellipsis length, a raw slice (no ellipsis) is returned. This path is untested; chosen as a reasonable fallback without expanding scope.

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