M11 Planning: Full Skins & Themes implementation

[csharpfritz]

M11 Planning: Full Skins & Themes Implementation

This issue tracks the full implementation of Skins & Themes support, building on the M10 PoC (issues #364-#368).

Scope (pending PoC results):

• StyleSheetTheme vs Theme priority mode distinction
• .skin file parser for direct migration of existing Web Forms theme files
• Sub-component style theming (HeaderStyle, RowStyle, AlternatingRowStyle on GridView, DetailsView, etc.)
• CSS file bundling from theme folders
• Container-level EnableTheming propagation to children
• Runtime theme switching
• JSON theme format as alternative configuration input
• Migration tooling integration (converter from .skin files to C# config, synergy with M12 tool)
• Documentation: migration guide for Web Forms themes to Blazor

Prerequisites:

• M10 PoC complete and validated (issues Skins & Themes PoC: ThemeConfiguration data model #364-Skins & Themes PoC: bUnit tests for theming pipeline #368)
• Jeff's answers to open questions in planning-docs/SKINS-THEMES-POC-PLAN.md section 6

Reference:

• planning-docs/SKINS-THEMES-COMPATIBILITY-REPORT.md (full research)
• planning-docs/SKINS-THEMES-POC-PLAN.md (PoC plan and architecture)