feat(code): replace diff editor with unified review panel (#1410)

## Problem

- current diff viewing experience is rough
- we have a ton of cloud vs. local branching logic that shouldn't _really_ be necessary

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## Changes

| review panel | split view | deferred rendering |
| --- | --- | --- |
| ![Screenshot 2026-03-31 at 6.04.35 PM.png](https://app.graphite.com/user-attachments/assets/49ba2693-6872-45c6-a1b9-841fd103d928.png)<br><br> | ![Screenshot 2026-03-31 at 8.16.04 PM.png](https://app.graphite.com/user-attachments/assets/7cbcbe6f-c855-4fc5-bb11-2b7616506d76.png)<br> | ![Screenshot 2026-03-31 at 8.16.13 PM.png](https://app.graphite.com/user-attachments/assets/c0d09315-45d5-4b9b-9040-b54564c5e0ac.png)<br><br>![Screenshot 2026-03-31 at 8.16.20 PM.png](https://app.graphite.com/user-attachments/assets/51ee75a8-892f-4c91-9033-666b466bf700.png)<br> |
|  |  |  |

- adds a new "review" panel that renders all diffs in one scrollable view
    - swaps codemirror for `@pierre/diffs` ([diffs.com](https://diffs.com/)) - shadow dom rendering w/ [web workers](https://diffs.com/docs#worker-pool) 🤓
    - defers large diffs (generated, deleted, just big)
- removes a ton of codemirror stuff that is no longer needed
- sorta-kinda-almost unifies the rendering for cloud and local diffs
    - local and cloud have seperate diff views, but both share as much as possible via `ReviewShell`
    - removed the separate `diff` and `cloud-diff` panel types/logic, uses a single `review` tab now

## next

- this is pretty fast because we're using web workers and only making a single git query to get the initial diff content for all files. HOWEVER it could be FASTER... i want to use [diffs' built-in virtualization](https://diffs.com/docs#virtualization), but could not get it to work yet, so i'll follow up on that
- [#1414] will probably swap the in-chat diff for this new view, then we will only need codemirror for full file viewing (and hopefully editing at some point...)
    - i asked the pierre team and they are planning to make diffs editable to some extent... 👀
- adding a "comment" button per line in the diff that allows you to "ask for followups" w/ that file ref

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## How did you test this?

manually, a bunch

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Author: adboio

apps/code/package.json

@@ -125,6 +125,7 @@
     "@opentelemetry/semantic-conventions": "^1.39.0",
     "@parcel/watcher": "^2.5.6",
     "@phosphor-icons/react": "^2.1.10",
+    "@pierre/diffs": "^1.1.7",
     "@posthog/agent": "workspace:*",
     "@posthog/electron-trpc": "workspace:*",
     "@posthog/git": "workspace:*",