feat(tui-go): idea board viewer

OverlayIdeas (opened with I, global/cross-session) lists the idea board in the
artifact-viewer shell: ↑↓ to move, x/d to remove (sends DiscardIdea, optimistic
drop), esc to close. Pull-based via ListIdeas -> idea.list.

apps/tui-go/internal/app/server.go

@@ -316,6 +316,12 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		m.stats = msg.Stats
 		m.statsFor = msg.SessionID
 		m.statsLoading = false
+	case protocol.TypeIdeaList:
+		m.ideas = msg.Ideas
+		m.ideasLoading = false
+		if m.ideasIndex >= len(m.ideas) {
+			m.ideasIndex = 0
+		}
 	case protocol.TypeConfigSnapshot:
 		m.configFields = msg.ConfigFields
 		m.configRestart = msg.ConfigRestartRequired
@@ -360,7 +366,8 @@ func sessionIDOf(msg protocol.ServerMessage) string {
 		protocol.TypeProtocolError, protocol.TypeProviderStatus,
 		protocol.TypeWorkflowList, protocol.TypeRouterResponse,
 		protocol.TypeModelChanged, protocol.TypeModelList, protocol.TypeResourceStatus,
-		protocol.TypeArtifactList, protocol.TypeConfigSnapshot, protocol.TypeSessionStats:
+		protocol.TypeArtifactList, protocol.TypeConfigSnapshot, protocol.TypeSessionStats,
+		protocol.TypeIdeaList:
 		return ""
 	default:
 		return msg.SessionID