feat: add support for stat sheet SVG rendering

Introduces the ability to discover and render `*.sheet.svg` files.
These files can contain `<text>` elements with `${key}` templates
that reactively update based on the current journal stats.

- Add `StatSheet` type and CLI scanning logic
- Implement `SheetView` for rendering SVGs with live bindings
- Add `parseSheet` utility to extract template patterns from SVG
- Update file indexer to include `.svg` files

src/components/journal/stat-sheet.ts

@@ -0,0 +1,64 @@
+/**
+ * Stat sheet template engine — parses *.sheet.svg files and extracts
+ * <text> elements containing ${key} template patterns.
+ *
+ * The SVG element is parsed once via DOMParser. The caller (StatsView)
+ * reactively updates the text nodes whenever the stats store changes.
+ */
+
+/** Regex matching ${key} template patterns (captures the key name). */
+const TEMPLATE_RE = /\$\{(\w+)\}/g;
+
+/** A single template binding in a <text> element. */
+export interface TextTemplate {
+  el: SVGTextElement;
+  template: string;
+  keys: string[];
+}
+
+/** The result of parsing a stat sheet SVG string. */
+export interface ParsedSheet {
+  svgElement: SVGSVGElement;
+  templates: TextTemplate[];
+}
+
+/**
+ * Parse an SVG string into a live DOM element and extract all text
+ * elements that contain ${key} template patterns.
+ */
+export function parseSheet(svgString: string): ParsedSheet {
+  const parser = new DOMParser();
+  const doc = parser.parseFromString(svgString, "image/svg+xml");
+  const svgElement = doc.documentElement as unknown as SVGSVGElement;
+
+  if (!svgElement || svgElement.tagName !== "svg") {
+    return {
+      svgElement: document.createElementNS(
+        "http://www.w3.org/2000/svg",
+        "svg",
+      ) as SVGSVGElement,
+      templates: [],
+    };
+  }
+
+  const templates: TextTemplate[] = [];
+  const textEls = svgElement.querySelectorAll("text");
+
+  for (const el of textEls) {
+    const content = el.textContent;
+    if (!content) continue;
+
+    TEMPLATE_RE.lastIndex = 0;
+    const keys: string[] = [];
+    let m: RegExpExecArray | null;
+    while ((m = TEMPLATE_RE.exec(content)) !== null) {
+      keys.push(m[1]);
+    }
+
+    if (keys.length > 0) {
+      templates.push({ el: el as SVGTextElement, template: content, keys });
+    }
+  }
+
+  return { svgElement, templates };
+}