Add recurring expenses with active nav tab highlighting

- Add RecurringExpense model to Prisma schema with migration
- Add lib/recurring-expenses.ts: CRUD + virtual projection per month
- Add /recurring-expenses API routes (GET, POST, PATCH, DELETE)
- Merge projected recurring expenses into dashboard totals and expense list
- Add RecurringExpenseManager component to /add-expense page
- Show amber "Recurring" badge on projected items; hide edit/delete for them
- Highlight active nav tab using usePathname() with hover state
- Fix Turbopack/Prisma stub issue by adding serverExternalPackages to next.config.ts
- Clear stale Turbopack stub in Dockerfile before each build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -8,7 +8,7 @@ COPY prisma ./prisma
 RUN npm ci
 
 COPY . .
-RUN npm run prisma:generate && npm run build
+RUN npm run prisma:generate && rm -rf .next/node_modules && npm run build
 
 FROM node:22-bookworm-slim AS runner
 WORKDIR /app