feat(frontend): implement Subscription Plan Manager UI

[delightonyedikachipixel]

Closes #399

Summary
This PR implements the Subscription Plan Manager UI within the dashboard settings. It provides users with a clear overview of their current subscription, their usage against limits, and available tier options.

Changes Proposed

• Created frontend/app/dashboard/settings/subscription/page.tsx.
• Added mock data for current usage and available subscription tiers (Free, Pro, Enterprise).
• Implemented a current plan summary card featuring a dynamic usage progress bar that turns red when approaching limits (>85%).
• Added a responsive grid for available plans, featuring dynamic button states (Current Plan, Upgrade, or Downgrade) based on the user's active tier limits.
• Styled the UI entirely with Tailwind CSS, fully supporting both light and dark modes.

Acceptance Criteria Checklist

• UI renders accurately based on mock user data.
• Current plan and limits are clearly displayed.
• Upgrade and downgrade buttons adapt correctly to the active tier.

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