PM-5378: Allow Design shortening across phases

[jmgasper]

What was broken
The previous implementation only allowed Design-track shortening on the currently open phase. QA clarified that Design challenges should allow any incomplete phase to be shortened, and active non-Design updates could still fail when Work sent a stale duration even though the saved scheduled end date did not move earlier.

Root cause
The PM-5378 validation guard was tied to phase.isOpen instead of the active challenge schedule as a whole. The challenge update helper also had a row-id-aware matcher but still used phase definition ids in the main update path, so row-specific timeline edits could be matched incorrectly.

What was changed
Design challenges can now shorten any incomplete phase as long as the requested end date is not before the current date/time. Active non-Design challenges guard every incomplete phase against shortening, while pre-launch non-Design reductions remain allowed. Full challenge updates and direct phase PATCHes both pass the active-challenge guard options, and phase update matching now prefers challenge phase row id before phase definition id.

Any added/updated tests
Bootstrapped the phase helper unit test and added regressions for Design future-phase shortening, active non-Design future-phase rejection, pre-launch non-Design shortening, and stale duration payloads that keep the persisted end date.

Validation:

• pnpm exec mocha test/unit/phase-helper.test.js --exit passed.
• pnpm lint passed.
• pnpm test is blocked in this checkout because DATABASE_URL is not set for Prisma-backed tests.
• pnpm build is unavailable because this package has no build script.