feat(frontend): Svelte editor, live preview and Git panel

- CodeMirror 6 editor with a debounced Mermaid 11 preview (zoom, pan,
  auto fit-to-view and inline error reporting)
- project start screen with recent projects, sidebar diagram explorer,
  toolbar and a Git panel (status, commit, history, branches)
- SVG/PNG export, per-project theme switching, toasts
- rune-based central store orchestrating all backend calls
- view modes (Code / Split / Preview) plus a viewer mode with a
  slide-in quick-edit drawer (Cmd/Ctrl+E)

src/lib/mermaid.ts

@@ -0,0 +1,35 @@
+import mermaid from 'mermaid';
+
+let currentTheme = '';
+
+function configure(theme: string) {
+  if (theme === currentTheme) return;
+  currentTheme = theme;
+  mermaid.initialize({
+    startOnLoad: false,
+    theme: theme as any,
+    securityLevel: 'loose',
+    fontFamily: 'inherit',
+    flowchart: { useMaxWidth: true, htmlLabels: true },
+  });
+}
+
+let renderSeq = 0;
+
+export type RenderResult = { svg: string } | { error: string };
+
+/** Render Mermaid source to an SVG string, capturing syntax errors. */
+export async function renderMermaid(code: string, theme: string): Promise<RenderResult> {
+  configure(theme);
+  if (!code.trim()) return { svg: '' };
+  const id = `mmx-render-${++renderSeq}`;
+  try {
+    const { svg } = await mermaid.render(id, code);
+    return { svg };
+  } catch (e) {
+    return { error: e instanceof Error ? e.message : String(e) };
+  } finally {
+    // Mermaid may leave a stray measurement node behind on failure.
+    document.getElementById(`d${id}`)?.remove();
+  }
+}