update subscription panel for new designs (#6378)

## Summary

Refactoring of subscription panel to improve maintainability and match
Figma design exactly. Extracted business logic into
`useSubscriptionCredits` and `useSubscriptionActions` composables, added
comprehensive testing, and enhanced the design system with proper
semantic tokens.

- Extract credit calculations and action handlers into reusable
composables
- Add component and unit tests with proper mocking patterns  
- Update terminology from "API Nodes" to "Partner Nodes"
- Make credit breakdown dynamic using real API data instead of hardcoded
values
- Add semantic design tokens for modal card surfaces with light/dark
theme support
- Reduce component complexity from ~100 lines to ~25 lines of logic
- Improve layout spacing, typography, and responsive behavior to match
Figma specs

<img width="1948" height="1494" alt="Selection_2220"
src="https://github.com/user-attachments/assets/b922582d-7edf-4884-b787-ad783c896b80"
/>

<img width="1948" height="1494" alt="Selection_2219"
src="https://github.com/user-attachments/assets/50a9f263-9adb-439d-8a89-94a498d394e3"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6378-update-subscription-panel-for-new-designs-29b6d73d3650815c9ce2c5977ac7f893)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/design-system/src/css/style.css

@@ -157,6 +157,8 @@
   --button-surface: var(--color-white);
   --button-surface-contrast: var(--color-black);
 
+  --modal-card-button-surface: var(--color-smoke-300);
+
   /* Code styling colors for help menu*/
   --code-text-color: rgb(0 122 255 / 1);
   --code-bg-color: rgb(96 165 250 / 0.2);
@@ -256,6 +258,8 @@
   --button-active-surface: var(--color-charcoal-600);
   --button-icon: var(--color-smoke-800);
 
+  --modal-card-button-surface: var(--color-charcoal-300);
+
   --dialog-surface: var(--color-neutral-700);
 
   --interface-menu-component-surface-hovered: var(--color-charcoal-400);
@@ -328,6 +332,7 @@
   --color-button-icon: var(--button-icon);
   --color-button-surface: var(--button-surface);
   --color-button-surface-contrast: var(--button-surface-contrast);
+  --color-modal-card-button-surface: var(--modal-card-button-surface);
   --color-dialog-surface: var(--dialog-surface);
   --color-interface-menu-component-surface-hovered: var(
     --interface-menu-component-surface-hovered