fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. Suite now passes 60/60 with no test files modified and no new dependencies.

Changes

• src/calculator.ts — divide now throws Division by zero when the divisor is 0 instead of returning Infinity.
• src/date-utils.ts — day-difference now uses Math.round instead of Math.floor, so 36 hours correctly reports "2 days ago" (fixes the off-by-one).
• src/string-utils.ts — implemented truncate: truncates at a word boundary with "..." counting toward maxLength, returns the string unchanged when str.length <= maxLength.
• src/task-manager.ts — completed the missing/incomplete TaskManager methods without mutating internal state on read.
• src/validator.ts — relaxed the isEmail TLD regex to accept long TLDs (e.g. .museum); reworked isUrl to use the platform URL parser with an http/https protocol guard (accepts ports like http://localhost:3000).

Verification

• bun test → 60 pass, 0 fail (70 assertions).
• Reviewed for correctness (not test-gaming) by the review agent.

Assumptions / Notes

• Only files under src/ were changed; no test files touched, no dependencies added.
• Reviewer flagged a pre-existing cosmetic quirk in date-utils.ts (23h30m–23h59m rounds to "24 hours ago" rather than "1 day ago"). It predates this change, is untested, and is out of scope — noted for a possible follow-up.