feat: use yjs as the source-of-truth

[lihebi]

This is a major change. Close #331

Before

Previous architecture (detailed in #205) is centered around the browser (yjs <-> browser <-> DB), which is problematic because:

1. we have two (potential inconsistent) sources of data (yjs and DB)
2. there can be multiple users/browsers. It's complex to decide which user/browser should push to DB

After

Now we are changing to Yjs-centered (browser <-> Yjs <-> DB). Yjs is the single source-of-truth.

Details

This PR removes a lot of syncing logic/functions from front-end, notably those addPod, loadRepo, setPodGeo reducers.

TODOs:

• [x] Better indicator to show the syncing status (e.g., synced, offline), prompt user unsynced changes when user leave the page.
• [x] When a user gets disconnected, edits can still be made, but these new edits might be lost. This is a bug. We need to figure out a graceful way to re-sync when Yjs WS re-connects.

[lihebi]

This feature is roughly usable. I'm merging this so that people can work on top of this, and avoid merge conflicts, as this PR is too large and touches everything.

Remaining problems: state.pods is now empty. So the following things break and need fixes in the follow-up PRs:

• [x] pod.content access, e.g., runtime, parser
• [x] pod.result access, e.g., display result, stdout/stderr, images
• [ ] export .ipynb and markdown
• [x] copy & paste

[lihebi]

Note: DB schema changed. You need to apply the migration and recreate your docker stack.