feat: add subscriptions list view

[wheval]

Summary

Adds the merchant subscriptions list page at /subscriptions with a responsive grid of plan cards and a primary action to create a new plan.

Purpose / Motivation

Merchants need a dedicated surface to view and manage recurring subscription plans. Issue #35 establishes the main subscriptions management route using the existing SubscriptionPlanCard component.

Changes Made

• Create Subscriptions List View #35: Create Subscriptions List View
• Added src/app/subscriptions/page.tsx with dashboard shell (sidebar + topbar), page header, and demo seed plans.
• Renders multiple SubscriptionPlanCard components in a responsive CSS grid (1 → 2 → 3 columns).
• Primary Create New Plan button at the top (UI placeholder; create flow not in scope for this issue).

How to Test

1. Run npm run dev and open /subscriptions.
2. Confirm the page title, description, and Create New Plan button appear at the top.
3. Verify four plan cards render with name, amount/token, frequency, and subscriber counts.
4. Resize the viewport: cards should stack on narrow screens, two columns from sm, three columns from xl.
5. Run npm run build and npm run typecheck to confirm no compile errors.

Breaking Changes

• None.

Related Issues

Closes #35

Checklist

• Code builds successfully (TypeScript passes)
• Tests added/updated
• No console errors
• Documentation updated (if needed)

[drips-wave]

@wheval Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[wheval]

@codebestia Please review

[codebestia]

LGTM!
Thanks