Allow administrators to submit some custom CSS

[csharpfritz]

Administrators should be able to add some extra css markup that will be included in all pages on SharpSite.

Should also be possible to add a LINK to a custom CSS resource

[csharpfritz]

Scope & Sanitization Decision (Confirmed)

User Input: v0.8.0 should support BOTH site-wide and per-page custom CSS, with sanitization that restricts unsafe/XSS-prone rules.

Translated to Requirements:

1. Site-wide CSS: Administrators can define global CSS rules applied to all pages
2. Per-page CSS: Administrators can override/extend CSS for individual pages
3. Sanitization: CSS rules blocked: script injection vectors, XSS-prone pseudo-selectors (e.g., :visited history leak), and arbitrary @-rules (@import, @font-face, @Keyframes)
   • Allowed: Basic selectors, colors, spacing, typography, layout (flexbox/grid)
   • Blocked: Expressions, calc(), IDOR via data URIs, external resource loads

Routing: @squad:simon Unblocks UI/form design for CSS editor component.

Impact: Tier 1 blocker cleared; Simon can scope CSS editor + sanitizer integration.