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
@@ -47,7 +48,8 @@ export default async function DashboardPage() {
 
   return (
     <div>
-      <div className="mb-8">
+      <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>
@@ -55,6 +57,8 @@ export default async function DashboardPage() {
             Select a project to open the flowchart editor
           </p>
         </div>
+        <NewProjectButton />
+      </div>
 
       {projects && projects.length > 0 ? (
         <div className="grid grid-cols-1 gap-4 sm:grid-cols-2 lg:grid-cols-3">

src/app/signup/page.tsx

@@ -8,19 +8,14 @@ import { createClient } from '@/lib/supabase/client'
 export default function SignupPage() {
   const router = useRouter()
   const searchParams = useSearchParams()
-  const [email, setEmail] = useState('')
+  // Pre-fill email if provided in URL (from invite link)
+  const [email, setEmail] = useState(searchParams.get('email') ?? '')
   const [password, setPassword] = useState('')
   const [confirmPassword, setConfirmPassword] = useState('')
   const [error, setError] = useState<string | null>(null)
   const [loading, setLoading] = useState(false)
 
   useEffect(() => {
-    // Pre-fill email if provided in URL (from invite link)
-    const emailParam = searchParams.get('email')
-    if (emailParam) {
-      setEmail(emailParam)
-    }
-
     // Handle invite/signup token from URL hash
     // Supabase adds tokens to the URL hash after redirect
     const handleTokenFromUrl = async () => {

src/components/NewProjectButton.tsx

@@ -0,0 +1,156 @@
+'use client'
+
+import { useState } from 'react'
+import { useRouter } from 'next/navigation'
+import { createClient } from '@/lib/supabase/client'
+
+export default function NewProjectButton() {
+  const [isOpen, setIsOpen] = useState(false)
+  const [projectName, setProjectName] = useState('')
+  const [isLoading, setIsLoading] = useState(false)
+  const [error, setError] = useState<string | null>(null)
+  const router = useRouter()
+
+  const handleOpen = () => {
+    setIsOpen(true)
+    setProjectName('')
+    setError(null)
+  }
+
+  const handleClose = () => {
+    if (!isLoading) {
+      setIsOpen(false)
+      setProjectName('')
+      setError(null)
+    }
+  }
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault()
+
+    if (!projectName.trim()) {
+      setError('Project name is required')
+      return
+    }
+
+    setIsLoading(true)
+    setError(null)
+
+    const supabase = createClient()
+
+    const {
+      data: { user },
+    } = await supabase.auth.getUser()
+
+    if (!user) {
+      setError('You must be logged in to create a project')
+      setIsLoading(false)
+      return
+    }
+
+    const { data, error: insertError } = await supabase
+      .from('projects')
+      .insert({
+        user_id: user.id,
+        name: projectName.trim(),
+        flowchart_data: { nodes: [], edges: [] },
+      })
+      .select('id')
+      .single()
+
+    if (insertError) {
+      setError(insertError.message || 'Failed to create project')
+      setIsLoading(false)
+      return
+    }
+
+    if (data) {
+      router.push(`/editor/${data.id}`)
+    }
+  }
+
+  return (
+    <>
+      <button
+        onClick={handleOpen}
+        className="inline-flex items-center gap-2 rounded-lg bg-blue-600 px-4 py-2 text-sm font-medium text-white transition-colors hover:bg-blue-700 focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 dark:focus:ring-offset-zinc-900"
+      >
+        <svg
+          className="h-5 w-5"
+          fill="none"
+          viewBox="0 0 24 24"
+          stroke="currentColor"
+        >
+          <path
+            strokeLinecap="round"
+            strokeLinejoin="round"
+            strokeWidth={2}
+            d="M12 4v16m8-8H4"
+          />
+        </svg>
+        New Project
+      </button>
+
+      {isOpen && (
+        <div className="fixed inset-0 z-50 flex items-center justify-center">
+          <div
+            className="absolute inset-0 bg-black/50"
+            onClick={handleClose}
+            aria-hidden="true"
+          />
+          <div className="relative w-full max-w-md rounded-lg bg-white p-6 shadow-xl dark:bg-zinc-800">
+            <h2 className="text-lg font-semibold text-zinc-900 dark:text-zinc-50">
+              Create New Project
+            </h2>
+            <p className="mt-1 text-sm text-zinc-600 dark:text-zinc-400">
+              Enter a name for your new visual novel project.
+            </p>
+
+            <form onSubmit={handleSubmit} className="mt-4">
+              <label
+                htmlFor="projectName"
+                className="block text-sm font-medium text-zinc-700 dark:text-zinc-300"
+              >
+                Project Name
+              </label>
+              <input
+                type="text"
+                id="projectName"
+                value={projectName}
+                onChange={(e) => setProjectName(e.target.value)}
+                placeholder="My Visual Novel"
+                disabled={isLoading}
+                autoFocus
+                className="mt-1 block w-full rounded-md border border-zinc-300 bg-white px-3 py-2 text-zinc-900 placeholder-zinc-400 focus:border-blue-500 focus:outline-none focus:ring-1 focus:ring-blue-500 disabled:cursor-not-allowed disabled:opacity-50 dark:border-zinc-600 dark:bg-zinc-700 dark:text-zinc-50 dark:placeholder-zinc-500"
+              />
+
+              {error && (
+                <p className="mt-2 text-sm text-red-600 dark:text-red-400">
+                  {error}
+                </p>
+              )}
+
+              <div className="mt-6 flex justify-end gap-3">
+                <button
+                  type="button"
+                  onClick={handleClose}
+                  disabled={isLoading}
+                  className="rounded-md border border-zinc-300 bg-white px-4 py-2 text-sm font-medium text-zinc-700 transition-colors hover:bg-zinc-50 focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 dark:border-zinc-600 dark:bg-zinc-700 dark:text-zinc-300 dark:hover:bg-zinc-600 dark:focus:ring-offset-zinc-800"
+                >
+                  Cancel
+                </button>
+                <button
+                  type="submit"
+                  disabled={isLoading}
+                  className="rounded-md bg-blue-600 px-4 py-2 text-sm font-medium text-white transition-colors hover:bg-blue-700 focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-2 disabled:cursor-not-allowed disabled:opacity-50 dark:focus:ring-offset-zinc-800"
+                >
+                  {isLoading ? 'Creating...' : 'Create Project'}
+                </button>
+              </div>
+            </form>
+          </div>
+        </div>
+      )}
+    </>
+  )
+}