Keep new tab button beside tabs / 固定新建会话按钮贴近标签页

[SivanCola]

Summary

• Keep the themed desktop tab list sized to its tab content before it shrinks.
• Preserve a stable new-session button slot immediately after the last visible tab.
• Add AppChrome CSS contract coverage so the themed tab list cannot grow into empty chrome space again.

Root cause

The themed AppChrome tab list used flex: 1 1 auto, so it expanded across the remaining titlebar space and pushed the + new-session button toward the right chrome controls when the right dock changed the available width.

Verification

• npm run check:css
• tsx src/__tests__/app-chrome-tabs.test.ts
• Browser DOM check confirmed the tab list no longer flex-grows and the + button sits beside the last tab.

Related

• Fixes [Feature]: 新建会话和搜索框会随着右边侧栏内容打开与合并进而发生位置变化，这个能固定住吗 #4483
• Related to fix(desktop): remove duplicate '+' button in TabBar #3986, which proposes removing the top + button entirely. This PR keeps the button and fixes its placement for the current UI direction.

[SivanCola]

Supersession note: #3986 explored the alternate product direction of removing the duplicate top + new-session button. For the current UI direction, this PR keeps the top + affordance and fixes its placement instead.

Thank you @NkAntony777 for the work in #3986: it clarified the duplicate-entry concern and helped frame the final product decision here.

Authorship note: @SivanCola is the primary author of this PR. @NkAntony777 contributed as a repository collaborator through #3986; that contribution is acknowledged as part of the shared repository work behind this final tab/new-session direction.