fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. The full suite now passes: 60 pass / 0 fail. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") on a zero divisor instead of returning Infinity.
• src/date-utils.ts — formatRelative now rounds the day count (Math.round) so an exact 36-hour delta reads as "2 days ago", matching the test's expectation.
• src/string-utils.ts — implemented wordCount handling of multiple consecutive spaces; implemented truncate word-boundary logic (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 ids), update (title/priority, false for unknown ids), and sortBy (priority high>medium>low, and createdAt oldest-first).
• src/validator.ts — isEmail now accepts long TLDs; isUrl now accepts URLs with a port (e.g. http://localhost:3000). Removed a stale BUG: JSDoc note that no longer applied.

Verification

• bun test → 60 pass, 0 fail, 70 expect() calls.
• Diff scoped to the 5 source files only; no test files touched.

Assumptions

• The 36-hour "2 days ago" result is intentional per the test, so Math.round (not Math.floor) is the correct rounding for formatRelative.
• update mutates the task in place (it holds the same reference stored in the Map), which is consistent with the existing add/get semantics.

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