feat(tui): add broot-style tab affordances to header and footer

Header now renders stream/cards as tabs with keybindings inline —
active tab highlighted, inactive shows the key to switch. Footer
shows a capture tab affordance when in list state. Redundant mode
and capture hints removed from the context hint bar.

internal/tui/model.go

@@ -786,16 +786,13 @@ func (m model) listContent() string {
 }
 
 func (m model) headerView() string {
-	header := titleStyle.Render("nib")
-
-	modeName := "stream"
-	if m.mode == modeCards {
-		modeName = "cards"
-	}
-	header += " " + modeStyle.Render(modeName)
+	header := titleStyle.Render("nib") + "  "
+	header += renderTab("stream", "1", m.mode == modeStream)
+	header += " " + separatorStyle.Render("│") + " "
+	header += renderTab("cards", "2", m.mode == modeCards)
 
 	if m.filterTag != "" {
-		header += " " + filterPillStyle.Render("#"+m.filterTag)
+		header += "  " + filterPillStyle.Render("#"+m.filterTag)
 	}
 
 	if m.hasSearch() {