Fix email detail pane height after resume

[mickn]

What changed

• make the email-detail scroll viewport a min-height-safe flex column
• allow the final expanded thread message to consume the remaining pane height
• prevent the analysis/classification row from shrinking
• add Electron regressions for both short and overflowing messages

Why

The message stack and expanded card were sized only by intrinsic content. After Exo returned from the background into a taller restored viewport, a short email could leave unused space below the classification row instead of filling the pane.

The fix completes the flex-height chain from the scroll viewport through the final message while preserving normal scrolling for long messages.

User impact

Short emails now fill the available reading pane after background/restore, with the classification row anchored to the bottom. Long emails remain scrollable.

Checks

• npm run build
• npm run typecheck:web
• playwright test tests/e2e/email-detail-layout.spec.ts --project=e2e — 2 passed