Unified class-based styling, PathIcon icon set and SearchBox redesign

[hendrikmennen]

Adds one shared, class-based design layer on top of Avalonia SimpleTheme in OneWare.Essentials, and moves the views over to it so the IDE and plugins look the same.

Design system (src/OneWare.Essentials/Styles/)

• OneWareTheme.axaml is the entry point, included once from OneWare.Core/App.axaml. Plugins get it from the host.
• Tokens: Tokens.axaml plus TokensDark.axaml / TokensLight.axaml. They keep the SimpleTheme key names and add status colors, radii, spacing, font sizes and control and icon sizes. The dark theme keeps the original OneWare palette.
• Controls/*.axaml holds one style file per control family. The Buttons, Borders, TextBoxes, ComboBoxes, ListBox, TreeView, TabControl, Menu, ScrollViewer and Expander styles moved here from OneWare.Core/Styles.
• Class vocabulary:
  • text: h1–h4, muted, caption, …
  • buttons: primary, secondary, ghost, danger, link, icon, …
  • surfaces: card, panel, inset, badge, callout, …
  • layout: toolbar, form, form-row, …
• Inputs get a default Height of 24 instead of a MinHeight, so an inline Height on a view still wins.
• Menus: submenu and dropdown popups now use the same elevated surface as context menus, and the gap between icon and text is smaller.

Icons

• Styles/Icons.axaml adds 152 monochrome Icon.* geometries. They are scaled to a 24×24 frame with consistent optical size.
• A new PathIcon theme makes icons inherit Foreground, so they follow button, hover and selection colors. Classes: small, large, muted, accent, success, warning, error.
• The old single-color DrawingImage keys still exist as thin wrappers, so IconModel and C# callers keep working. Multi-color icons are unchanged.
• About 100 view usages are converted from <Image Source=…> to <PathIcon Data=…>.

SearchBox

• SearchBox is now a single TextBox with a leading search icon, a clear button that shows only when there is text, and a new IsBusy spinner.
• The extra wrapper borders around it are removed in ErrorList, PackageManager, ProjectExplorer, LibraryExplorer, SearchList and the PinPlanner.

Views

• Inline colors, borders, font sizes and local styles are replaced with classes across Core, ApplicationCommands, CloudIntegration, PackageManager, SourceControl, Settings, the FPGA project system, WaveFormViewer, SerialMonitor and others.
• Chat and Copilot views are intentionally unchanged.

Docs and tooling

• New docs/Styling.md covers tokens, classes, icons and SearchBox. docs/PluginDevelopment.md gets a Styling section.
• Help → Style Gallery (Debug builds only) shows every class and icon in dark and light side by side.

Testing

• dotnet build OneWare.slnx succeeds and dotnet test OneWare.slnx passes.
• The style gallery and several views were checked in headless renders.

Companion plugin PR: one-ware/OneWare.AI#739. It runs against the current Essentials package and uses DynamicResource Icon.* until the package version is bumped.