docs: update BACKLOG, SPEC, and design system to reflect v0.9.0–v0.16.3 changes

- BACKLOG: add completed features from v0.7.1 through v0.16.3 (shopping
  widget, priority none, kanban persistence, meal visibility, currency,
  custom categories, recipe links, Spanish/FR/TR/RU/EL/ZH locales,
  settings tabs, CNY/TRY/RUB currencies)
- SPEC: add recipe_url field to Meals table (v0.13.0)
- SPEC: add shopping_categories table (v0.12.0)
- SPEC: fix i18n fallback language from de to en (v0.16.1)
- SPEC: update locale file list to include all 10 languages
- SPEC: document Settings tab navigation (v0.16.0)
- design/system.md: fix priority levels from 4 to 5 (none added v0.9.0)

.interface-design/system.md

@@ -163,5 +163,5 @@ Two triggers: `prefers-color-scheme: dark` (system) or `data-theme="dark"` (manu
 - **Module-specific CSS:** Each page has its own stylesheet loaded dynamically - keeps initial bundle minimal
 - **Page transitions:** Directional slide animations based on navigation order (left/right)
 - **Glass overlays:** `rgba(255, 255, 255, 0.18)` surfaces for elements on colored backgrounds (e.g., action buttons on module headers)
-- **Priority system:** Four levels (urgent/high/medium/low) with dedicated color + translucent background tokens
+- **Priority system:** Five levels (none/low/medium/high/urgent) with dedicated color + translucent background tokens. "None" is the default for new tasks and hides the priority badge entirely.
 - **Swipe gestures:** Touch-friendly item interactions (swipe to delete/complete) on mobile