Provider connection dialog: header content (icon, title, close button) hugs the modal edges

[cijiugechu]

What happened

When opening Settings → Models and selecting a provider, the connection
dialog renders with the header content — provider icon, title, subtitle and
close button — flush against the modal edges with no padding.

This is not specific to the provider dialog: the same defect affects every
dialog (keyboard shortcuts help, bot onboarding, WeChat QR, MCP editor,
search, etc.).

Root cause: regression introduced by #1674 (72f5d981 "refactor(ui): move
modal layers to Astryx"). Before the migration, Maka's own DialogHeader
carried px-4 py-2.5 padding. The Astryx DialogHeader derives its inset
from the dialog container's --layout-padding-* variables, but Maka's
createModalContent defaults padding = 0, so the header loses all inset.

How to reproduce

1. Open Settings → Models
2. Click a provider to open its connection dialog
3. Observe the header: icon, title, subtitle and close button are flush against the dialog edges

Environment

• Maka version or commit: 0.1.2, main HEAD d07d4f1e (regression from
  72f5d981, refactor(ui): move modal layers to Astryx #1674)
• OS and version: macOS (Apple Silicon)
• Surface: Desktop
• Node.js version, if running from source: 24.18.1

Logs, screenshots, or additional context