feat: add maintenance-mode banner to showcase site

[AntonioVentilii]

Motivation

GIX Components is moving to maintenance mode. We want visitors to the showcase (gix.design) to see this immediately, so we add a site-wide notice banner.

Changes

• Add a slim, site-wide maintenance-mode banner at the top of the showcase layout (src/routes/+layout.svelte).
• Styled with the existing --warning-emphasis design tokens.
• Showcase app only — not shipped in the published package; no version impact.

Screenshots

To add: screenshot of the banner at the top of the showcase. Run npm run dev to preview.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to ffcf871.

Security Overview

• 🔎 Scanned files: 1 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/98bf2300-834e-44ab-93d0-06f69eae67e3?scanId=1878016f-4ff2-466b-b161-ba02ca638eb5&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► +layout.svelte     Add maintenance banner