fix: implement missing utils and correct 5 logic bugs

[stooit]

Summary

Fixes all 16 failing tests across the 5 source files. bun test now reports 60 pass / 0 fail (was 44 pass / 16 fail), and tsc --noEmit is clean. No test files or dependencies were modified.

Changes

• src/calculator.ts — divide now throws on division by zero (also covers -0) instead of returning Infinity.
• src/date-utils.ts — formatRelative day calc uses Math.round(Math.abs(diffHours) / 24), fixing the off-by-one so 36h reports "2 days ago" (past and future symmetric).
• src/validator.ts — isEmail TLD quantifier widened {2,4} → {2,} (accepts long TLDs like .museum); isUrl drops the empty-port requirement so http://localhost:3000 validates, while keeping the protocol allow-list.
• src/string-utils.ts — wordCount uses trim().split(/\s+/) for consecutive spaces; truncate implemented with ellipsis counted toward maxLength and word-boundary cutting.
• src/task-manager.ts — implemented remove (returns boolean), update (guarded key-wise assignment, returns boolean), and sortBy (priority and createdAt ordering on a sorted copy, preserving insertion order via stable sort).

Verification

• bun test → 60 pass / 0 fail
• tsc --noEmit → exit 0
• Diff reviewed: no correctness blockers found.

Assumptions / notes

• Scoped strictly to what the tests require (per task constraints). Review flagged a few untested edge cases (negative maxLength, clearing an optional description, sortBy with an unrecognised field) — left out of scope as no test exercises them and the task said to fix only what tests require.
• isUrl is a format validator, not an SSRF guard — documented in its docstring.