fix: prevent scroll jump when undo banner dismisses on mobile

[jgordijn]

Summary

• Undo banner was positioned inline in document flow, causing the entries list to shift when the banner auto-dismissed after 15 seconds
• On mobile, this shift was visible as a scroll jump since the banner was often below the viewport
• Changed to a fixed-position toast at the bottom of the viewport so it never affects document flow

Test plan

• Mark an article as read on mobile — undo banner appears at bottom of screen
• Wait 15s for auto-dismiss — no scroll jump occurs
• Verify banner looks correct on desktop (centered, max-width constrained)
• Test undo button still works