WIP: Rewrite export-html with tree sidebar, client-side rendering

- Add tree sidebar with search and filter (Default/All/Labels)
- Client-side markdown/syntax highlighting via vendored marked.js + highlight.js
- Base64 encode session data to avoid escaping issues
- Reuse theme.ts color tokens via getResolvedThemeColors()
- Sticky sidebar, responsive mobile layout with overlay
- Click tree node to scroll to message
- Keyboard shortcuts: Esc to reset, Ctrl/Cmd+F to search

packages/coding-agent/src/modes/interactive/theme/theme.ts

@@ -652,6 +652,91 @@ export function stopThemeWatcher(): void {
 	}
 }
 
+// ============================================================================
+// HTML Export Helpers
+// ============================================================================
+
+/**
+ * Convert a 256-color index to hex string.
+ * Indices 0-15: basic colors (approximate)
+ * Indices 16-231: 6x6x6 color cube
+ * Indices 232-255: grayscale ramp
+ */
+function ansi256ToHex(index: number): string {
+	// Basic colors (0-15) - approximate common terminal values
+	const basicColors = [
+		"#000000",
+		"#800000",
+		"#008000",
+		"#808000",
+		"#000080",
+		"#800080",
+		"#008080",
+		"#c0c0c0",
+		"#808080",
+		"#ff0000",
+		"#00ff00",
+		"#ffff00",
+		"#0000ff",
+		"#ff00ff",
+		"#00ffff",
+		"#ffffff",
+	];
+	if (index < 16) {
+		return basicColors[index];
+	}
+
+	// Color cube (16-231): 6x6x6 = 216 colors
+	if (index < 232) {
+		const cubeIndex = index - 16;
+		const r = Math.floor(cubeIndex / 36);
+		const g = Math.floor((cubeIndex % 36) / 6);
+		const b = cubeIndex % 6;
+		const toHex = (n: number) => (n === 0 ? 0 : 55 + n * 40).toString(16).padStart(2, "0");
+		return `#${toHex(r)}${toHex(g)}${toHex(b)}`;
+	}
+
+	// Grayscale (232-255): 24 shades
+	const gray = 8 + (index - 232) * 10;
+	const grayHex = gray.toString(16).padStart(2, "0");
+	return `#${grayHex}${grayHex}${grayHex}`;
+}
+
+/**
+ * Get resolved theme colors as CSS-compatible hex strings.
+ * Used by HTML export to generate CSS custom properties.
+ */
+export function getResolvedThemeColors(themeName?: string): Record<string, string> {
+	const name = themeName ?? getDefaultTheme();
+	const isLight = name === "light";
+	const themeJson = loadThemeJson(name);
+	const resolved = resolveThemeColors(themeJson.colors, themeJson.vars);
+
+	// Default text color for empty values (terminal uses default fg color)
+	const defaultText = isLight ? "#000000" : "#e5e5e7";
+
+	const cssColors: Record<string, string> = {};
+	for (const [key, value] of Object.entries(resolved)) {
+		if (typeof value === "number") {
+			cssColors[key] = ansi256ToHex(value);
+		} else if (value === "") {
+			// Empty means default terminal color - use sensible fallback for HTML
+			cssColors[key] = defaultText;
+		} else {
+			cssColors[key] = value;
+		}
+	}
+	return cssColors;
+}
+
+/**
+ * Check if a theme is a "light" theme (for CSS that needs light/dark variants).
+ */
+export function isLightTheme(themeName?: string): boolean {
+	// Currently just check the name - could be extended to analyze colors
+	return themeName === "light";
+}
+
 // ============================================================================
 // TUI Helpers
 // ============================================================================