SPA tests: pure utils — tokenStore, dates.todayIso, ageWeeks #109

Follow-up to #105 / PR #106 — small pure helpers, easy wins.

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• auth/tokenStore.ts — save→load round-trip; clearTokens empties the slot; loadTokens on corrupt JSON calls removeItem and returns null; missing key → null.
• lib/dates.ts todayIso() — returns farm-local YYYY-MM-DD (not UTC); pin with fake timers around an evening-UTC-rollover instant to prove it does not roll to the next day. Zero-pads month/day.
• routes/FlocksPage.tsx ageWeeks (Math.max(0, Math.floor(days/7))) — extract to a testable util and cover 0/6/7/13 days + negative (pre-placement) → 0.

Part of #105, epic #14.