Add spacing before code review branch picker

[moirahuang]

Description

Adds external left spacing to the legacy code review branch picker in the wide header row so it does not crowd the diff stat.

The diff stat already has a right margin, and the branch picker already has internal horizontal button padding. This keeps the picker’s internal padding and hover target unchanged, and adds a small left margin to the picker wrapper instead.

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Slack request: feedback-app message 1779472497.944709

Testing

• cargo check --manifest-path /workspace/warp/Cargo.toml -p warp
• I have manually tested my changes locally with ./script/run

Manual UI testing was not run in this sandboxed environment. rustfmt was also unavailable in the sandbox toolchain (rustfmt is not installed for stable-x86_64-unknown-linux-gnu), so I reviewed the small diff manually.

Screenshots / Videos

Not captured; change is a targeted spacing adjustment based on the attached Slack screenshot.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

Co-Authored-By: Oz oz-agent@warp.dev

Conversation: https://staging.warp.dev/conversation/d3a1cb82-a4f7-40d2-8684-8e206cb4e449
Run: https://oz.staging.warp.dev/runs/019e50d3-85ac-7d71-a8fd-d8e477444ba7
This PR was generated with Oz.