feat(shop): animated backgrounds — system-wide cosmetic + picker

A new cosmetic family: a fixed-position overlay painted behind every
page of the app, switchable from the profile shop. 4 free presets + 6
paid (250-1200 coins) so the new economy has another sink. Every
animation respects prefers-reduced-motion and falls back to its static
gradient.

Catalogue (migration 035):
  free:   none, gradient-soft, dots, dark
  paid:   gradient-flow, grid, bubbles, stars (mid)
          aurora, nebula                       (premium)

Backend:
  • migration 035 adds users.active_background + rebuilds shop_items
    CHECK to include 'background' (standard SQLite 'new + copy + swap')
    and seeds 10 items
  • shopController.getMyActive returns { background: { slug } } and
    activateItem handles type='background' (stores bare slug in
    active_background) + skips the user_purchases check for price=0
    so free presets work for everyone without per-user rows
  • routes/shop validate schema lets 'background' through

Frontend:
  • api.js applyCosmetics injects <div id='ls-bg-fx'> at body start
    and toggles class to bg-<slug>. Cleared backgrounds remove the
    element so dark→light transitions don't leave artifacts.
  • ls.css gains a self-contained 'ANIMATED BACKGROUNDS' block:
    keyframes per animated slug (ls-bg-flow, ls-bg-grid-scan,
    ls-bg-bubble-rise, ls-bg-stars-twinkle, ls-bg-aurora-spin,
    ls-bg-nebula-pan) wrapped in a prefers-reduced-motion kill-switch.
    Same .bg-<slug> classes are reused for the .bg-preview swatches.
  • profile.html shop:
    - new 'Фоны' filter button between Рамки and Титулы
    - _renderItemPreview type='background' draws a real 56-aspect swatch
      (same CSS as the page bg — what you see is what you apply)
    - _isItemActive matches by slug for background type
    - free items (price===0) treated as auto-owned in render so users
      can apply them without a fake 'purchase' step

Verified: getMyActive returns { background: { slug: 'nebula' } } after
flipping users.active_background; activate path updates the row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/routes/shop.js

@@ -18,10 +18,10 @@ function shopGate(req, res, next) {
 }
 
 const purchaseLimiter = rateLimit({ windowMs: 60_000, max: 10, message: 'Слишком много покупок, подождите минуту' });
-const activateSchema  = { body: { type: { type: 'string', oneOf: ['frame', 'title', 'effect'] } } };
+const activateSchema  = { body: { type: { type: 'string', oneOf: ['frame', 'title', 'effect', 'background'] } } };
 const adminItemSchema = { body: {
   name:  { type: 'string', required: true, minLen: 1, maxLen: 200 },
-  type:  { type: 'string', required: true, oneOf: ['frame', 'title', 'effect'] },
+  type:  { type: 'string', required: true, oneOf: ['frame', 'title', 'effect', 'background'] },
   price: { type: 'number', required: true, min: 0 },
 }};
 const awardCoinsSchema = { body: {