feat: add updatedAt field; order projects by last update time

[lihebi]

Now each project (repository) will record the updatedAt and createdAt fields. The project list will be ordered based on last update time, with most recent showing first.

[lihebi]

Update:

1. Use "last viewed" instead of "last updated" to sort the projects, the same behavior as Google Docs.
2. In DB schema, the data is stored in a UserRepoData table with one entry for each <user,repo> pair, so that each user owns a different setting for each repo.

model UserRepoData {
    id         String   @id
    user       User     @relation(fields: [userId], references: [id])
    userId     String
    repo       Repo     @relation(fields: [repoId], references: [id])
    repoId     String
    accessedAt DateTime @default(now()) @updatedAt
    dummyCount Int      @default(0)
}