Add Symbol and SymbolPosition to web buttons

Web Button controls currently show text only, while desktop buttons can display the curated SystemSymbol catalogue. Developers building Web projects cannot show a Save, Add, Delete, Search, or Refresh glyph without supplying their own image assets.

Add system-symbol support to web buttons:

• Button.Symbol As SystemSymbol — a semantic system symbol shown on the button, or SystemSymbol.None for no symbol.
• Button.SymbolPosition As SymbolPosition — where the symbol sits relative to the label: Leading (default), Trailing, Above, or Below.

Symbols can be assigned from code and selected in the Web page designer's inspector using the shared symbol picker. They persist in Web page layouts and are emitted from generated project code. In the browser they render as scalable vectors that inherit the current foreground colour, so they stay crisp at any DPI and respond to themes, disabled state, and selection.