Add landing page, redesign emails, and return updated task on completion

- Integrate landing page into Go app (served at root /)
- Add STATIC_DIR config for static file serving
- Redesign all email templates with modern dark theme styling
- Add app icon to email headers
- Return updated task with kanban_column in completion response
- Update task DTO to include kanban column for client-side state updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/router/router.go

@@ -51,6 +51,20 @@ func SetupRouter(deps *Dependencies) *gin.Engine {
 	r.Use(corsMiddleware(cfg))
 	r.Use(i18n.Middleware())
 
+	// Serve landing page static files (if static directory is configured)
+	staticDir := cfg.Server.StaticDir
+	if staticDir != "" {
+		r.Static("/css", staticDir+"/css")
+		r.Static("/js", staticDir+"/js")
+		r.Static("/images", staticDir+"/images")
+		r.StaticFile("/favicon.ico", staticDir+"/images/favicon.svg")
+
+		// Serve index.html at root
+		r.GET("/", func(c *gin.Context) {
+			c.File(staticDir + "/index.html")
+		})
+	}
+
 	// Health check endpoint (no auth required)
 	r.GET("/api/health/", healthCheck)