feat: [US-013] - Create new project

Add NewProjectButton component with modal dialog for creating new projects:
- Button displays on dashboard with plus icon
- Modal with project name input
- Creates project in Supabase with empty flowchart_data
- Redirects to /editor/[projectId] on success
- Error handling with user feedback

Also fixes lint error in signup page (setState in effect).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/dashboard/page.tsx

@@ -1,5 +1,6 @@
 import { createClient } from '@/lib/supabase/server'
 import Link from 'next/link'
+import NewProjectButton from '@/components/NewProjectButton'
 
 interface Project {
   id: string
@@ -47,13 +48,16 @@ export default async function DashboardPage() {
 
   return (
     <div>
-      <div className="mb-8">
-        <h1 className="text-2xl font-bold text-zinc-900 dark:text-zinc-50">
-          Your Projects
-        </h1>
-        <p className="mt-1 text-sm text-zinc-600 dark:text-zinc-400">
-          Select a project to open the flowchart editor
-        </p>
+      <div className="mb-8 flex items-start justify-between">
+        <div>
+          <h1 className="text-2xl font-bold text-zinc-900 dark:text-zinc-50">
+            Your Projects
+          </h1>
+          <p className="mt-1 text-sm text-zinc-600 dark:text-zinc-400">
+            Select a project to open the flowchart editor
+          </p>
+        </div>
+        <NewProjectButton />
       </div>
 
       {projects && projects.length > 0 ? (