feat(ui): implement reusable metric cards and skeleton loaders for dashboard optimization

[DebasmitaBose0]

Summary

Enhances the main frontend dashboard by introducing reusable components (MetricCard) and smooth pulsing loaders (SkeletonLoader) to mitigate layout shifts and improve visual aesthetics during slow database or telemetry network fetches.

Proposed Changes

• [NEW] SkeletonLoader.tsx : Declares a reusable loading element displaying a modern pulsing linear-gradient shimmer layout.
• [NEW] MetricCard.tsx : Extracts metrics rendering into a modular, reusable container component capable of rendering standard metrics or transitioning to skeleton-loader states.
• [NEW] dashboard_components.css : Implements glassmorphism card backgrounds, transition shifts, color indicators for positive/negative trends, and custom shimmer keyframe animations.
• [NEW] dashboard_demo.tsx : Implements a standalone demo page illustrating the loading states and the responsive metric-card grid setup.
• [MODIFY] index.tsx : Integrates the Next.js router instance and hooks up a navigation link in the main header to access the demo view.

Verification Plan

Verified that the Next.js application compiles and loads routes successfully.

Closes #519

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📥 Commits

Reviewing files that changed from the base of the PR and between 4361d01 and 1e09e04.

📒 Files selected for processing (5)

• frontend/components/MetricCard.tsx
• frontend/components/SkeletonLoader.tsx
• frontend/pages/dashboard_demo.tsx
• frontend/pages/index.tsx
• frontend/styles/dashboard_components.css

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• Create PR with unit tests

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🧪 PR Test Results

Check	Result
Tests (pytest tests/)	✅ success
Lint (ruff check .)	❌ failure
Coverage (agentwatch)	73.43%

_{Python 3.12 · commit 1e09e04}