Show cached patches as blue boxes on the map

[aivanchenk]

Adds a third overlay layer that visualizes which patches are currently held in the in-memory LRU cache, so you can see the real footprint of what's been downloaded as you click around — not just the single active patch.

The colour story

The map now reads as a cool teal → blue → violet ramp, each hue a distinct role:

Overlay	Colour	Meaning
Cell	teal	the single pixel you clicked
Patch	violet (#51)	the 40×40 chunk this click just downloaded
Cached	blue	every distinct lat/lon patch still in the LRU cache

Blue boxes render beneath the violet patch and teal cell, so the active selection always stays on top.

How it works

• ZarrChunkReader now exposes getCachedPatches() (distinct lat/lon patches, de-duplicated across time chunks) plus a subscribe() / getCacheVersion() pair, so the map updates reactively — including when background prefetch completes.
• chunkIndicesToPatchBounds() extracted from chunkPatchBounds() to turn chunk indices straight into geo bounds.
• A cache toggle (stacked-squares icon) appears in the nav once anything is cached, mirroring the patch toggle.

Verify

• eslint, next build (incl. its TypeScript pass), and vitest (44 tests, +1 for chunkIndicesToPatchBounds) all pass.
• Not yet visually confirmed in a running browser — the overlay geometry/colours should be eyeballed on a real click.

Base

Stacked on 38-patch-extent (#51) because it reuses that PR's violet patch tokens. Retarget to main once #51 merges.

[gemini-code-assist]

Code Review

This pull request introduces a feature to visualize cached patches on the map, adding a toggle button in the navigation bar, updating the map overlay to render cached bounds, and implementing a subscription mechanism in ZarrChunkReader to track cache mutations. The review feedback highlights a fragile cache key parsing implementation that could fail if variable names contain colons, and suggests batching cache change notifications using queueMicrotask to prevent rapid, redundant re-renders during concurrent chunk loads.

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[github-actions]

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