fix(gui): Storage workspace UI and policy chrome

[Wibias]

Summary

• Port Storage rail/overview workspace (less nested containers, less forced scroll)
• Rescan confirmation feedback in the page head (no layout shift)
• Run now always clickable; auto-enables cleanup when needed
• Policy saved / errors sit beside Run now (not under Save)
• Drop Oldest beside the cleanup percent
• Policy form fields + NumberStepper align with Codex Auth chrome

Test plan

• Storage page opens as workspace (rail + overview/detail)
• Rescan shows confirmation without shifting layout
• Run now works when policy is off (enables then runs)
• Save policy: feedback appears next to Run now
• Percent label has no Oldest prefix
• Steppers / toggles / selects match Codex Auth styling

Part of the GUI UI/UX split (UI-3). Companion draft: #607 (UI-1 chrome).

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