Match empty view minimum width

[ComputelessComputer]

Apply the note surface minimum width rules to empty tabs and cover the body and window guard behavior with layout tests.

Stack

• Base: main
• Follow-up: Keep sidebar collapse from restoring window width #5843 fix/sidebar-collapse-window-width builds on this PR's shared window-width guard.

Changes

• Share the note-surface minimum width rule so empty and session tabs reserve the same 500px content surface.
• Apply that rule in the classic body panel and main chat panel window-width guard.
• Cover empty-tab body/chat panel layout behavior with regression tests.

Verification

• pnpm -F desktop exec vitest run src/shared/main/chat-panels.test.tsx
• pnpm -F desktop typecheck

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