Design polish: collapsed composer, space headers, member cues

The capture box now starts as a quiet "Add a Memory…" row that expands when you click it (and collapses after you capture). Shared Space headers now show your role with a clear badge — Owner, Editor, or Viewer — and new members get a short "Getting started here" card. Viewer access is now spelled out where capture isn't available, Owners are marked with a shield, and your own row shows a "You" tag. Copy tightened throughout.