feat: create repo without a name; generate userId and repoId from server

[lihebi]

Now, the "create new project" button will directly drop you into a new project without asking for a name. The project name is editable on the top navbar. Screenshot:

This PR also move the userId and repoId generation to the server. The nanoID format is changed a little, from prefixed 10-bit nolookalikes (e.g., repo_6e66FcLE3K) to 20-bit lowercase + numbers (e.g., sh360e667a5gffk16441).

[lihebi]

Now, the "create new project" button will directly drop you into a new project without asking for a name.

@li-xin-yi let's move "make-a-repo-public" to a "share" button in the top-right corner of the canvas, just like Google docs & Feishu docs (I'll work on this).