Polish web v2 navigation and visual tokens

[roackb2]

Summary

• split the web-v2 sidebar into focused navigation structure components
• keep the current v2 scope limited to Sessions, Tasks, and settings
• centralize v2 panel, nav row, control, popover, resize-handle, sidebar spacing, and sidebar toggle visual language in Tailwind CSS
• apply the shared visual language to dropdowns, popovers, panel headers, dividers, resize handles, and navigation rows
• add a desktop sidebar collapse/expand toggle backed by the existing resizable panel primitive

Verification

• yarn client:v2:build
• yarn lint
• yarn typecheck
• yarn test:browser-integration:v2
• Playwright computed-style smoke check for popover/control/panel borders and sidebar collapse/alignment

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: e26f91d952

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".