feat: buy-cycle consumption prediction for untracked products

Products like salt/spices that are never marked per-use now get
consumption rate estimated from the average time between restocks:
  avgCycleDays = (lastIn - firstIn) / (buyCount - 1)
  estimatedDaysLeft = avgCycleDays - daysSinceLastBuy

Requirements: buyCount >= 3, dailyRate == 0, avgCycle >= 7 days.
Appears in smart shopping list with reason 'Finisce tra ~Ngg (ciclo medio Mgg)'.
Also marks buy-cycle products as isRegular so stock checks apply.

CHANGELOG.md

@@ -11,6 +11,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - **Recipe scraps tips** — During cooking steps, detect "waste" generated (peels, cores, bones, eggshells, coffee grounds, citrus zest, etc.) and surface AI-powered tips on how to reuse them (compost, natural cleaner, broth, candied peel, etc.). Could be shown as an optional collapsible hint card below the step that generates the scrap.
 
+## [1.7.29] - 2026-05-29
+
+### Added
+- **Buy-cycle consumption prediction** — Products that are never tracked per-use (salt, spices, cleaning supplies, etc.) now use the average time between restocks as a proxy for consumption rate. When a product has ≥ 3 purchase events and no individual `out` events, EverShelf calculates the average buy cycle (`(lastBuy - firstBuy) / (buyCount - 1)`) and estimates how many days of stock remain in the current cycle. The product appears in the smart shopping list with a reason like "Finisce tra ~12gg (ciclo medio 75gg)" before it runs out, rather than only after. These products are now also treated as `isRegular` so all stock-level urgency checks apply correctly.
+
+
 ## [1.7.28] - 2026-05-30
 
 ### Fixed