fix: implement missing utilities and fix edge-case bugs (all tests green)

[stooit]

Summary

Fixes all failing tests in the utility library. The suite went from 44 pass / 16 fail to 60 pass / 0 fail, tsc --noEmit exits clean, and only the five source files were touched — no test files modified, no dependencies added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when b === 0 (also catches -0) instead of silently returning Infinity.
• src/string-utils.ts
  • wordCount — split on /\s+/ against the trimmed string so runs of consecutive whitespace (and tabs/newlines) collapse; fixes "hello world" returning 3.
  • truncate — implemented to the documented contract: unchanged when str.length <= maxLength; otherwise reserves 3 chars for "...", snaps to the last word boundary, result always <= maxLength. Negative/small-limit guard clamped with Math.max(0, maxLength).
• src/task-manager.ts — implemented the three stubbed methods: remove (returns delete boolean), update (partial update; description clearable via "description" in changes), and sortBy (priority / status / createdAt, stable for ties).
• src/date-utils.ts — diffDays now does Math.round(Math.abs(diffHours) / 24), matching the abs-then-round convention of the adjacent branches. Removes the off-by-one and the past/future asymmetry.
• src/validator.ts
  • isEmail — accepts arbitrary subdomain depth and long TLDs, still rejects whitespace / missing @ / malformed labels.
  • isUrl — dropped the empty-port condition that rejected http://localhost:3000; still restricted to http/https, requires a non-empty hostname.

Verification

• bun test -> 60 pass, 0 fail
• tsc --noEmit -> exit 0
• Independent review pass returned a ship verdict; its priority finding (signed-rounding asymmetry) plus two safe hardening items were applied before commit.

Assumptions

• Implemented each function to the contract expressed by its tests and in-source TODO:/BUG: comments.
• divide(x, 0) throwing is an intentional breaking change vs. the old Infinity behaviour — pinned by the test.
• isEmail is a format-level check, not authoritative validation.

Follow-ups (non-blocking)

• isEmail now silently rejects underscores and non-ASCII/IDN domains (previously accepted) — worth a docstring note.
• A cleared description retains the key with undefined; delete task.description would be cleaner if tasks are ever persisted.