fix: pass all failing tests (implement missing functions + fix edge cases)

[stooit]

Summary

Fixes all 16 failing tests across 5 files. Test suite now: 60 pass / 0 fail, tsc --noEmit clean. No test files modified, no dependencies added.

Changes by file

• src/calculator.ts — divide throws Error("Division by zero") when divisor is 0 (was silently returning Infinity/NaN).
• src/string-utils.ts
  • wordCount — trims and splits on /\s+/ so runs of whitespace no longer count as extra words.
  • truncate — implemented per the TODO spec: returns unchanged if str.length <= maxLength; otherwise reserves 3 chars for "...", cuts to budget, and backs up to the last word boundary. Falls back to a hard cut when the first word exceeds the budget.
• src/task-manager.ts — implemented stubbed methods: remove (returns delete boolean), update (returns false for unknown id; applies only provided keys), sortBy (priority high→medium→low; createdAt oldest first).
• src/date-utils.ts — diffDays uses Math.round instead of Math.floor, so 36h now reads "2 days ago".
• src/validator.ts — isEmail accepts long TLDs ({2,}); isUrl accepts URLs with a port.

Assumptions & notes

• divide throwing is a behavioural change for any caller that relied on Infinity — driven by the test.
• isUrl now accepting arbitrary ports widens accepted input; if it ever guards outbound requests, port-restriction may be worth revisiting (SSRF surface). Out of scope for these tests.
• createdAt sort relies on spec-stable Array.prototype.sort so same-ms ties keep insertion order.

Testing

• bun test — 60 pass / 0 fail
• tsc --noEmit — clean