feat: Add SWR local storage cache layer for offline dashboard resiliency

[dhruvil-1207]

Fixes #2801

Description

Implemented a lightweight client-side data resiliency layer using a Stale-While-Revalidate (SWR) model for the core dashboard widgets.

This ensures that if a developer works on a spotty local network or encounters GitHub API rate-limits, the dashboard avoids crashing or showing endless loading spinners by instantly hydrating from a local snapshot while syncing live data in the background.

Changes Made

• PRMetrics, StreakTracker, and GoalTracker: Intercepted incoming API dashboard metrics payloads.
• Added localStorage caching with timestamps.
• Implemented immediate UI hydration on load using the saved snapshot.
• Added silent background fetching to fetch live data and overwrite the stale snapshot without disrupting the user layout.
• Handled errors gracefully so the dashboard doesn't crash if localStorage is completely empty or disabled.

Testing

• Tested locally by simulating an "Offline" network in Chrome DevTools.
• Verified that widgets instantly display cached data on hard refresh.
• Confirmed that failed background requests are quietly swallowed, preventing UI crashes.

[github-actions]

GSSoC Label Checklist 🏷️

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