fix: implement missing utilities and fix edge-case bugs

[stooit]

Summary

Fixes all 16 failing tests across 5 source files. All 60 tests now pass; no test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws on division by zero instead of returning Infinity.
• src/string-utils.ts — wordCount collapses consecutive whitespace (splits on \s+ after trim). truncate implemented: truncates at a word boundary with "..." counting toward maxLength, returns unchanged when str.length <= maxLength, and handles short/edge inputs.
• src/task-manager.ts — implemented remove (true/false on found), update (partial title/description/priority, false for unknown id), and sortBy (priority high>medium>low, createdAt oldest-first, status workflow order). sortBy sorts a copy and does not mutate internal state.
• src/date-utils.ts — formatRelative now rounds days instead of flooring, so 36h reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs (bound widened to DNS label limit {2,63}) while still rejecting empty labels; isUrl accepts URLs with ports, keeping the http/https protocol allow-list.

Assumptions

• Expected behaviour was derived from the existing test files; no test was modified.
• sortBy("status") uses the workflow order pending -> in_progress -> completed (no explicit test asserts this ordering, chosen as the natural default).

Security note

isUrl validates scheme only. If ever used to gate a server-side fetch it is not sufficient SSRF protection on its own — http://localhost:3000 and link-local addresses pass. A host allow-list / private-range block at the call site would be needed for that use case.

Verification

bun test -> 60 pass, 0 fail. tsc --noEmit clean.