feat(dashboard): optimal stale-while-revalidate cache for system stats

[OlziYT]

This PR extracts the stale-while-revalidate cache from the bugfix PR #3024 into its own dedicated feature, using Svelte 5 store patterns to ensure state is safely encapsulated.

Changes

• Introduces system-stats-cache.store.svelte.ts which provides a clean API for caching system metrics with a configurable TTL (currently 5 minutes).
• Refactors dashboard-all-environments-view.svelte to read from and write to this store instead of a module-level variable.

This provides the optimal UX by preventing skeleton flashes during menu navigation, while respecting SvelteKit architecture best practices.

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Greptile Summary

This PR adds stale-while-revalidate caching for dashboard system stats. The main changes are:

• Adds a context-scoped systemStatsCacheStore with TTL-based cached stats.
• Initializes the cache context from the app layout.
• Seeds dashboard live stats from cached values and refreshes them from WebSocket updates.

Confidence Score: 5/5

The changes are focused on dashboard stats caching and do not introduce identified merge-blocking concerns.

The reviewed changes are small, localized, and align with the stated cache behavior without identified correctness or security issues.

T-Rex Logs

What T-Rex did

• Attempted to start the app environment by running docker-compose up -d --build, but the command failed because docker was not found.
• The setup attempt was repeated, and it again failed at the same step due to the missing Docker engine.
• Because the app server never started, no valid before/after UI video or dashboard screenshot could be produced.
• A cache contract probe was run to evaluate head versus base cache behavior, showing that base initially had no system-stats-cache.store.svelte.ts and no dashboard cache references, and after the probe the head passes the cache contract with expected results: same-env cache hit, different-env null, a valid timestamp at 5 minutes, null after TTL, context store reuse, and dashboard cached state initialized with loading false and hasLoaded true, with fresh stats overwriting the cache.

_{Ran code and verified through T-Rex}

_{Reviews (2): Last reviewed commit: "fix(dashboard): address greptile code re..." | Re-trigger Greptile}

[OlziYT]

@greptileai