feat: Add Project Management feature

- Add Project, ProjectMember, ProjectInvite models to Prisma schema
- Add API routes for projects CRUD, members management, invite links
- Add ProjectSelector and ProjectSettings components
- Add /projects, /projects/[id]/settings, /invite/[token] pages
- Update Navbar with navigation links

prisma/schema.prisma

@@ -23,9 +23,11 @@ model User {
   createdAt     DateTime  @default(now())
   updatedAt     DateTime  @updatedAt
 
-  accounts Account[]
-  sessions Session[]
-  tasks    Task[]
+  accounts        Account[]
+  sessions        Session[]
+  tasks           Task[]
+  ownedProjects   Project[]       @relation("OwnedProjects")
+  projectMembers  ProjectMember[]
 
   @@map("users")
 }
@@ -74,6 +76,7 @@ model VerificationToken {
 model Task {
   id          String   @id @default(cuid())
   userId      String
+  projectId   String?  // Optional - cho backward compatibility
   title       String
   description String?
   status      String   @default("TODO") // TODO, IN_PROGRESS, DONE
@@ -81,10 +84,62 @@ model Task {
   createdAt   DateTime @default(now())
   updatedAt   DateTime @updatedAt
 
-  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+  user    User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  project Project? @relation(fields: [projectId], references: [id], onDelete: Cascade)
 
   @@index([userId])
+  @@index([projectId])
   @@index([status])
   @@index([deadline])
   @@map("tasks")
 }
+
+// ==================== PROJECT MODELS ====================
+
+model Project {
+  id          String   @id @default(cuid())
+  name        String
+  description String?
+  ownerId     String
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  owner   User            @relation("OwnedProjects", fields: [ownerId], references: [id], onDelete: Cascade)
+  members ProjectMember[]
+  invites ProjectInvite[]
+  tasks   Task[]
+
+  @@index([ownerId])
+  @@map("projects")
+}
+
+model ProjectMember {
+  id        String   @id @default(cuid())
+  projectId String
+  userId    String
+  role      String   @default("MEMBER") // OWNER, ADMIN, MEMBER
+  joinedAt  DateTime @default(now())
+
+  project Project @relation(fields: [projectId], references: [id], onDelete: Cascade)
+  user    User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([projectId, userId])
+  @@index([userId])
+  @@map("project_members")
+}
+
+model ProjectInvite {
+  id        String   @id @default(cuid())
+  projectId String
+  token     String   @unique @default(cuid())
+  email     String?
+  expiresAt DateTime
+  used      Boolean  @default(false)
+  createdAt DateTime @default(now())
+
+  project Project @relation(fields: [projectId], references: [id], onDelete: Cascade)
+
+  @@index([token])
+  @@index([projectId])
+  @@map("project_invites")
+}