fix: stack impersonation banner with customer alerts

[HarshMN2345]

The header-alert system previously rendered only the highest-importance banner. Since the impersonation banner has importance 100, it always suppressed payment-failed and other billing alerts (importance 1), making them invisible to operators during impersonation sessions.

Now the impersonation banner renders in a separate fixed-position stack alongside the highest-priority customer alert, so operators can see exactly what the customer sees while impersonating.

What does this PR do?

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[greptile-apps]

Greptile Summary

This PR fixes the impersonation-banner-suppressing-billing-alerts problem by introducing an AlertStack wrapper that renders both banners in a single fixed-position container, with each HeaderAlert child delegating layout management to the parent stack via Svelte context. The headerAlert store's get() is refactored to use getStore (removing the spurious-subscriber anti-pattern) and gains getExcluding() to skip the importance-100 impersonation entry.

One cleanup gap remains: src/routes/+layout.svelte (outside the diff) still registers the impersonation banner in the store and reactively updates its show flag — code that is now dead because every consumer uses getExcluding('impersonation'). The getExcluding calls and the store registration are coupled; removing both together would simplify the design.

Confidence Score: 5/5

Safe to merge; the fix is correct and well-scoped, with only a minor cleanup gap remaining.

All findings are P2. The core logic — AlertStack layout management, isInStack context delegation, getExcluding exclusion — is sound. The only issue is leftover dead code in routes/+layout.svelte that is outside the diff and doesn't affect correctness.

src/routes/+layout.svelte (not in diff) — contains now-redundant headerAlert.add/updateShow calls for the impersonation entry.

Important Files Changed

Filename	Overview
src/lib/layout/alertStack.svelte	New fixed-position wrapper that manages layout height via ResizeObserver for stacked banners; duplicates setNavigationHeight logic from headerAlert.svelte, but cleanly delegates via isInStack context.
src/lib/layout/headerAlert.svelte	Correctly opts out of self-managed layout/observers when isInStack context is true; fixes bannerSpacing.set(null) type mismatch.
src/lib/stores/headerAlert.ts	Replaces update()-for-read anti-pattern with getStore(); adds getExcluding() to allow billing alerts to bypass the importance-100 impersonation entry; removes unused set() method.
src/lib/layout/shell.svelte	Wraps ImpersonationBanner and activeHeaderAlert in AlertStack; derives hasAnyAlert to gate the stack; impersonation visibility is derived independently of the headerAlert store.
src/lib/stores/billing.ts	Swaps headerAlert.get() for getExcluding('impersonation') so billing alerts are no longer shadowed by the importance-100 impersonation entry.
src/routes/(console)/+layout.svelte	Replaces get() with getExcluding('impersonation') in checkForUsageLimits and afterUpdate; the registration of the impersonation entry in headerAlert (in routes/+layout.svelte) was not removed, leaving that code as dead weight.
src/lib/layout/index.ts	Adds AlertStack to the layout barrel export.

_{Reviews (2): Last reviewed commit: "fix: address code review findings" | Re-trigger Greptile}