Refactor into multi-template monorepo with Next.js support

Restructure project to support multiple templates via Nix flakes and omnix.
Users can now choose between vanilla JavaScript or Next.js TypeScript
templates during initialization.

- Move existing template to templates/vanilla/
- Add templates/nextjs/ with App Router, TypeScript, and React 19
- Update flake.nix for multi-template outputs
- Add TypeScript support to shared devshell
- Preserve FHS compatibility for both templates
- Update README with comprehensive template comparison and usage

nix/modules/devshell.nix

@@ -22,6 +22,7 @@ in {
         [
           bun
           nodejs
+          typescript
           eslint
           just
           prettierd

nix/modules/template.nix

@@ -1,19 +1,41 @@
-{inputs, ...}: {
+{inputs, ...}: let
+  root = inputs.self;
+in {
   flake = rec {
-    templates.default = {
-      description = "A minimal JavaScript project template with Bun and Nix";
-      path = inputs.self;
+    templates = {
+      default = {
+        description = "Minimal JavaScript template with Vite and Bun";
+        path = "${root}/templates/vanilla";
+      };
+
+      nextjs = {
+        description = "Next.js TypeScript template with App Router for Vercel";
+        path = "${root}/templates/nextjs";
+      };
     };
 
-    om.templates.javascript-template = {
-      template = templates.default;
-      params = [
-        {
-          name = "package-name";
-          description = "Name of the JavaScript package";
-          placeholder = "javascript-template";
-        }
-      ];
+    om.templates = {
+      javascript-vanilla = {
+        template = templates.default;
+        params = [
+          {
+            name = "package-name";
+            description = "Name of the JavaScript package";
+            placeholder = "my-project";
+          }
+        ];
+      };
+
+      javascript-nextjs = {
+        template = templates.nextjs;
+        params = [
+          {
+            name = "package-name";
+            description = "Name of the Next.js project";
+            placeholder = "my-nextjs-app";
+          }
+        ];
+      };
     };
   };
 }