Agentic UI: Port selective sync

[nightnei]

Related issues

• Fixes STU-1946

How AI was used in this PR

Claude Code assisted with mapping the legacy selective-sync implementation via a few iterations. All changes were reviewed and visually verified by hand in both color schemes.

Proposed Changes

As agreed in STU-1946, the Agentic UI adopts the classic UI's Selective Sync as-is for the beta, so both UIs offer the same sync capabilities.
To keep review easy, the legacy modules were intentionally copied verbatim into apps/ui/src/components/selective-sync/.
Note, there is an existing issue in trunk - UI says that the Push is finished, hovewer it isn't. This issue is not covered in this PR - DOTPROD-170.

Testing Instructions

1. Open Agentic UI, connect a site to a local site
2. Click on Pull and Push and assert that the dialog looks completely the same as in Classic UI and selection works well in the tree

3. Try to pull all data or select for example only `themes` folder and assert that it works well. 4. You can test the push action too; it works, but there is an existing bug which is not covered in this PR (DOTPROD-170).

[shaunandrews]

We'll iterate on the UI more once this merges, but the only thing we should consider is adding a max-width to the modal to avoid these huge line-lengths:

[nightnei]

We'll iterate on the UI more once this merges, but the only thing we should consider is adding a max-width to the modal to avoid these huge line-lengths:

Limited to 960px

[wpmobilebot]

📊 Performance Test Results

Comparing 5949654 vs trunk

app-size

Metric	trunk	5949654	Diff	Change
App Size (Mac)	1406.34 MB	1406.37 MB	+0.03 MB	⚪ 0.0%

site-editor

Metric	trunk	5949654	Diff	Change
load	1015 ms	1069 ms	+54 ms	🔴 5.3%

site-startup

Metric	trunk	5949654	Diff	Change
siteCreation	7525 ms	7529 ms	+4 ms	⚪ 0.0%
siteStartup	2869 ms	2871 ms	+2 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[katinthehatsite]

😮

That is to say that I started the review and checking this now

[katinthehatsite]

What happens if I start the sync process in the new UI, then switch to the classic UI? So far, I am seeing no sync progress in the classic UI in that case and I can start again the sync from there? I think I expected the sync progress to be reflected even if I change between the UI mid-process. What do you think?

[katinthehatsite]

I think it would be nice if these links had color even if they don't in the old UI:

(the one at the end that has Create a new backup)

[katinthehatsite]

Another observation is that I was able to delete the local site mid-sync e.g. while pushing. Would the delete option be disabled when the push is happening to prevent sync errors?

[nightnei]

Another observation is that I was able to delete the local site mid-sync e.g. while pushing. Would the delete option be disabled when the push is happening to prevent sync errors?

This PR just covers porting sync functionality verbatim, so I would move this to a separate PR.

[nightnei]

I think it would be nice if these links had color even if they don't in the old UI:

(the one at the end that has Create a new backup)

I think it's a good point. Shoun has follow-up PRs to change the styles. This PR intentionally porting sync functionality verbatim, and it's already too big.

[nightnei]

What happens if I start the sync process in the new UI, then switch to the classic UI? So far, I am seeing no sync progress in the classic UI in that case and I can start again the sync from there? I think I expected the sync progress to be reflected even if I change between the UI mid-process. What do you think?

Hm, I tested it before and it was reflected in Classic UI. Thanks, I will double check it.
But I don't think it's critical, as the goal is ultimately to remove the Classic UI.

[bcotrim]

Backup node ids can contain commas — the rewind ls returns composite ids for themes (cjE6,ZjE6Lw==) and plugins (cjI6,ZjI6Lw==). Joining them here and splitting on , in the CLI flag corrupts the selection: pulling mu-plugins + themes + plugins sent 5 fragments instead of 3 ids to initiateBackup (verified against the real CLI argv). Classic sends this list as a JSON array directly to wpcom, so only this new desktop→CLI hop is affected.

[bcotrim]

LGTM, thanks for making the change 👍

I pushed another small fix so we can merge