[Fix] Keep Session start errors compact

[daniel-lxs]

What changed

A failed Session start used a page-sized empty-state component below the composer, making the New Session dialog unnecessarily tall and showing raw transport errors. Replace it with a compact inline row: a small warning icon, “Couldn’t start this session,” and Retry. The same shared form also improves the home composer.

The existing captured-request retry, stable conversation identity, duplicate-start guard, and validation dialog are unchanged. Follow-up to #3010.

Validation

• Inspected the actual New Session dialog after a controlled database start failure in light theme on desktop and at 390 × 844. The error row is 48px tall on desktop, with a readable message and visible Retry action on mobile.
• Restored the dependency, edited the draft, and clicked Retry on mobile. The dialog closed and the Session opened with the original captured prompt; the persisted message matched it.
• 39 focused Home, launcher, and NewTaskDialog tests passed, including the transport-error copy and original-request retry assertions.
• Web type checking, repository lint, formatting, and diff checks passed.

The browser test used disposable local fixtures. Model execution was outside its scope.

[roomote-community]

No code issues found. See task

_{Reviewed db694ab}