refactor: rename ManaDeck to Cards across entire monorepo

Rename the flashcard/deck management app from ManaDeck to Cards:
- Directory: apps/manadeck → apps/cards, packages/manadeck-database → packages/cards-database
- Packages: @manadeck/* → @cards/*, @manacore/manadeck-database → @manacore/cards-database
- Domain: manadeck.mana.how → cards.mana.how
- Storage: manadeck-storage → cards-storage
- Database: manadeck → cards
- All shared packages, infra configs, services, i18n, and docs updated
- 244 files changed, zero remaining manadeck references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/dependabot.yml

@@ -89,7 +89,7 @@ updates:
       - "automated"
 
   - package-ecosystem: "docker"
-    directory: "/apps/manadeck/apps/backend"
+    directory: "/apps/cards/apps/backend"
     schedule:
       interval: "weekly"
     open-pull-requests-limit: 5