feat: add SWR local storage cache layer for offline dashboard resiliency (#2801)

[yachikadev]

Summary

Implements a Stale-While-Revalidate (SWR) local storage cache layer for the dashboard's Streak Tracker and Goal Tracker widgets, so users see their last-known data instantly on load instead of a blank/loading state — even on spotty networks, during GitHub API rate-limiting, or fully offline.

Closes #2801

Changes

• src/lib/localCache.ts (new) — lightweight saveSnapshot / getSnapshot / clearSnapshot helpers backed by localStorage, with try/catch guards so a full, disabled, or missing localStorage never throws at runtime.
• src/components/StreakTracker.tsx — on mount, hydrates instantly from the last cached streak/contribution snapshot before the network call resolves. On successful fetch, persists the new snapshot. On fetch failure, falls back to the cached snapshot instead of showing an error.
• src/components/GoalTracker.tsx — same pattern applied to the goals list: instant hydration from cache, snapshot saved on successful load, cached goals shown if the live fetch fails.

How it works

1. Instant hydration — a useEffect on mount reads the cached snapshot from localStorage and populates state immediately, skipping the loading spinner if data is available.
2. Background sync — the existing fetch logic runs unchanged in parallel; on success it silently overwrites both the UI state and the cached snapshot.
3. Graceful fallback — if the live request fails (offline, rate-limited, etc.), the cached snapshot is shown instead of an error message, whenever one exists.

Notes

• Scoped to the two widgets driven by /api/metrics and /api/goals (streaks + goals), matching the issue's acceptance criteria. Other widgets are untouched and can adopt the same localCache.ts utility in future PRs if desired.

[github-actions]

GSSoC Label Checklist 🏷️

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