add ClideMarkdown, ClideCodeBlock, ClideSvgView primitives

Three reusable rendering widgets in lib/widgets/:

- ClideMarkdown: walks markdown AST (GFM tables, task lists,
  fenced code), renders headings, lists, blockquotes, tables,
  inline bold/italic/code/links. Delegates code blocks to
  ClideCodeBlock.

- ClideCodeBlock: syntax-highlighted code via TreeSitterService.
  Async highlight, byte-to-char offset mapping, theme-aware
  colors from SurfaceTokens.

- ClideSvgView: wraps jovial_svg with asset/string constructors
  and optional sizing.

Retrofitted: decision detail body renders as markdown instead of
raw text. Markdown viewer uses ClideMarkdown. Welcome logo uses
ClideSvgView.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_svg_view.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/services.dart' show rootBundle;
+import 'package:flutter/widgets.dart';
+import 'package:jovial_svg/jovial_svg.dart';
+
+class ClideSvgView extends StatelessWidget {
+  const ClideSvgView.asset(this.assetPath, {super.key, this.width, this.height}) : svgString = null;
+  const ClideSvgView.string(this.svgString, {super.key, this.width, this.height}) : assetPath = null;
+
+  final String? assetPath;
+  final String? svgString;
+  final double? width;
+  final double? height;
+
+  @override
+  Widget build(BuildContext context) {
+    Widget child;
+    if (assetPath != null) {
+      child = ScalableImageWidget.fromSISource(
+        si: ScalableImageSource.fromSvg(rootBundle, assetPath!),
+      );
+    } else if (svgString != null) {
+      final si = ScalableImage.fromSvgString(svgString!);
+      child = ScalableImageWidget(si: si);
+    } else {
+      return const SizedBox.shrink();
+    }
+
+    if (width != null || height != null) {
+      child = SizedBox(width: width, height: height, child: child);
+    }
+    return child;
+  }
+}