fix: repair edge-case bugs and implement missing utility functions

[stooit]

Summary

Fixes all 16 failing tests in the utility library (44 → 60 pass / 0 fail). Only the 5 source files were changed — no test files modified, no dependencies added, clean tsc --noEmit.

Changes

• calculator.ts — divide now throws on division by zero instead of returning Infinity.
• string-utils.ts — wordCount collapses consecutive whitespace ("hello world" → 2); implemented truncate with word-boundary logic where "..." counts toward maxLength, returning the string unchanged when within the limit and handling maxLength <= 3 without exceeding it.
• task-manager.ts — implemented remove (returns true/false), update (title/description/priority, returns true/false), and sortBy (priority high > medium > low, plus createdAt/status, relying on stable sort for insertion-order tiebreaks).
• date-utils.ts — fixed formatRelative off-by-one: 36h ago now rounds to "2 days ago"; rounding applied on the magnitude so past/future are symmetric.
• validator.ts — isEmail accepts long TLDs (e.g. .museum); isUrl accepts host-only and ported URLs (e.g. http://localhost:3000).

Verification

• bun test → 60 pass / 0 fail (70 expect() calls)
• npx tsc --noEmit → clean
• Reviewed by a review pass; three latent correctness issues surfaced (truncate length contract, formatRelative future-diff asymmetry, fragile numeric-id tiebreak) were all addressed even though no current test covers them.

Assumptions

• truncate spec (word boundary, ellipsis-in-budget) inferred from the test file's expected values and the source TODO comment.
• sortBy priority ordering (high > medium > low) taken from the test expectations.