fix(desktop): recover the embedded laser view when a project switch restarts the brain

[pyramation]

Summary

Switching projects while on Show left the embedded artist UI a black rectangle until the app was reloaded. It isn't z-index: the native WebContentsView was still there, sitting on a load that never completed.

Two causes, one shape — nothing ever asked again:

• invalidateLaserView() fires the reload right after the brain restarts. If that load fails (server not listening yet, or it is aborted by a laser:sync load racing it), the URL gate is cleared but the renderer has no reason to send another sync, so no code path retries.
• A view mid-load paints nothing, and it was setVisible(true) regardless, so the failure showed as black rather than as the route's own empty state.

So the view now tracks desired vs loaded separately and heals itself:

load(url)  → loadedUrl = url; painted = false
             loadURL(...).then(paint).catch(retryLater)
retryLater → setTimeout(load, min(2000, 250 * 2**attempts))   // only while desiredUrl === url
visible    ⇔ desiredUrl != null && painted

Leaving Show (syncLaser({visible:false})) clears desiredUrl, which cancels the pending retry — a backoff that outlives the screen it belongs to is the next bug. Each load takes a loadSeq, so the ERR_ABORTED rejection of a superseded load can't schedule a retry over the load that replaced it.

__tests__/laser-view.test.ts covers this against a mocked electron: paint-then-show, retry after a failed load, backoff growth, cancellation on leaving Show, reload on project switch, and no reload for an unchanged URL. Four of the six fail against the previous implementation.

Link to Devin session: https://app.devin.ai/sessions/972698f89f494b86828010666a002b8f
Requested by: @pyramation

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