fix(app): space review navigation groups

[opencode-agent]

Summary

• match the file-tree/navigation gap to the 12px review control-group spacing
• keep the previous/next arrows flush
• avoid additive spacing when collapsed review metadata is visible

Validation

• bun typecheck (packages/session-ui)
• bun test (packages/session-ui) — 59 passed
• bun typecheck (packages/app)
• Playwright visual geometry check: tree → arrows 0px → 12px; arrows remain 0px apart; right-side control groups remain 12px apart