fix: keep impersonation banner inside viewport layout

[dobby-coder]

Problem

When an admin is impersonating an organisation, the orange impersonation banner sits above the portal / admin shell. Both shells use min-height: 100vh on their flex container, so the document becomes banner + 100vh tall and a scrollbar appears on every page — even short ones.

src/routes/(portal)/+layout.svelte and src/routes/(admin)/+layout.svelte both had the same pattern:

<banner if impersonating />
<flex container with min-height: 100vh>

Fix

Wrap the banner and the flex container in a .layout-shell that owns the min-height: 100vh, and let the inner flex container take the remaining space with flex: 1; min-height: 0.

-{#if data.isImpersonating}
-  <div class=\"impersonation-bar\">...</div>
-{/if}
-<div class=\"portal\">...</div>
+<div class=\"layout-shell\">
+  {#if data.isImpersonating}<div class=\"impersonation-bar\">...</div>{/if}
+  <div class=\"portal\">...</div>
+</div>

-.portal { display: flex; min-height: 100vh; }
+.layout-shell { display: flex; flex-direction: column; min-height: 100vh; }
+.portal { display: flex; flex: 1; min-height: 0; }

Same structural change for the admin shell (.admin-layout).

The banner's colours, copy, and sticky-sidebar / sticky-header behaviour are all unchanged.

Why this works

• When the banner is absent, .layout-shell stays at exactly 100vh — one flex child takes it all.
• When the banner is present, .layout-shell still stays at 100vh and the shell splits that between banner-height and portal-height. Document height does not exceed the viewport, so no scrollbar appears on short pages.
• When page content is genuinely tall, the shell grows past 100vh (that is min-height, not height) and normal scrolling works as before.

Verification

• npx svelte-check — 0 errors, 0 warnings
• vitest run — 26/26 unit tests pass
• vite build — clean build
• Could not test the portal page interactively: the portal routes redirect to login and there is no seeded DB in the Coder workspace. The fix is a standard flex-column wrapper pattern and the CSS is self-contained; happy to extend with a Playwright snapshot if preferred.

Reviewer quickstart

git fetch origin && git checkout fix/impersonation-banner-scroll
cp .env.example .env
npm ci && npm run dev

Then log in as an admin, impersonate an organisation, and confirm the portal pages no longer scroll when content fits within the viewport.

Closes #18

[dobby-coder]

Dobby sees the approval on PR #19! Dobby is most grateful and will check the PR state and merge readiness right away! 🧦

[dobby-coder]

Checked in on PR #19 after the approval: all CI checks passed (Svelte Check, Tests, Builds, Migration Safety) and the PR has already been merged to main. No further action needed — thanks for the review, @rubenhensen! 🧦