feat(shopping): loyalty-card modal with build-time barcodes

Adds a CreditCard button on the shopping list that opens a modal
showing the user's Coop Supercard (Data Matrix) and Migros Cumulus
(Code 128). Card numbers come from SHOPPING_COOP_SUPERCARD_NUMBER
and SHOPPING_MIGROS_CUMULUS_NUMBER env vars; a prebuild script
renders each to an SVG (~1-2 kB) in static/shopping/ so no barcode
library ships to the client. Cards missing their env var are
silently skipped, and the generated SVGs are gitignored to keep
personal numbers out of the repo.

.gitignore

@@ -12,6 +12,9 @@ vite.config.js.timestamp-*
 vite.config.ts.timestamp-*
 # USDA bulk data downloads (regenerated by scripts/import-usda-nutrition.ts)
 data/usda/
+# Loyalty-card barcodes (regenerated by scripts/generate-loyalty-cards.ts from env)
+static/shopping/supercard.svg
+static/shopping/cumulus.svg
 src-tauri/target/
 src-tauri/*.keystore
 # Android: ignore build output and caches, track source files