fix(admin): center loading spinner under logo on login page

[all3f0r1]

What does this PR do?

The Kumo <Loader /> SVG wasn't visually centered under the logo on the login page loading state. Tailwind's Preflight sets SVGs to display: block, so text-align: center on the parent had no effect. Switched the wrapper to a flex column layout with items-center for reliable centering.

Closes #589

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm lint passes
• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• User-visible strings in the admin UI are wrapped for translation and pnpm locale:extract has been run (if applicable)
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: https://github.com/emdash-cms/emdash/discussions/...

AI-generated code disclosure

• This PR includes AI-generated code

Screenshots / test output

CSS-only change — the wrapper div switches from text-center (which doesn't center block-level SVGs) to flex flex-col items-center (which does).

[changeset-bot]

🦋 Changeset detected

Latest commit: 3bd9a86

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@emdash-cms/admin	Patch
emdash	Patch
@emdash-cms/cloudflare	Patch
@emdash-cms/auth	Patch
@emdash-cms/blocks	Patch
@emdash-cms/gutenberg-to-portable-text	Patch
@emdash-cms/x402	Patch
create-emdash	Patch
@emdash-cms/plugin-embeds	Patch

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commit: 3bd9a86