fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Makes all 60 tests pass (was 44 pass / 16 fail). Fixes edge-case bugs and implements missing functionality across the five utility modules. No test files modified, no dependencies added — changes are confined to src/.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity/NaN, failing loudly at the boundary.
• string-utils.ts — implemented truncate (truncates at a word boundary, "..." counts toward maxLength, returns unchanged when within limit, handles strings shorter than the ellipsis). Fixed wordCount to collapse consecutive whitespace (splits on /\s+/, also handles tabs/newlines).
• task-manager.ts — implemented remove (returns false for unknown id), update (title/priority; false for unknown id), and sortBy (priority: high→medium→low; createdAt: oldest first). sortBy copies before sorting so insertion order in list() is preserved.
• date-utils.ts — fixed off-by-one in formatRelative day rounding (Math.round instead of Math.floor, consistent with the minute/hour branches).
• validator.ts — isEmail accepts arbitrary-length TLDs and multi-level subdomains (and got stricter: rejects a@b..com, a@.com); isUrl accepts URLs with a port (removed an erroneous url.port === "" check) while retaining the http/https scheme allowlist.

Verification

60 pass / 0 fail — Ran 60 tests across 5 files

tsc --noEmit is clean.

Assumptions & known limitations

Per the task constraint ("fix only what the tests require"), the following untested edge cases were left as-is and are documented here for follow-up:

• date-utils.formatRelative uses Math.round on days; at exact half-day ties, past vs. future differ by direction (no test covers the future case).
• string-utils.truncate assumes a positive maxLength; negative/NaN values are unguarded.
• task-manager.update cannot clear the optional description field (undefined means "leave unchanged").
• validator.isUrl performs syntactic validation only — it should not gate server-side fetches without a host allowlist. isEmail is syntactic only and does not prove deliverability.