Display generation model in message header and other tweaks

Adjusted `ctrl+t` in chat view to toggle `showDetails` which toggles the
display of system messages, message metadata (generation model), and
tool call details

Modified message selection update logic to skip messages that aren't
shown

pkg/tui/views/chat/chat.go

@@ -89,11 +89,11 @@ type Model struct {
 	persistence     bool // whether we will save new messages in the conversation
 
 	// UI state
-	focus           focusState
-	wrap            bool     // whether message content is wrapped to viewport width
-	showToolResults bool     // whether tool calls and results are shown
-	messageCache    []string // cache of syntax highlighted and wrapped message content
-	messageOffsets  []int
+	focus          focusState
+	showDetails    bool     // whether various details are shown in the UI (e.g. system prompt, tool calls/results, message metadata)
+	wrap           bool     // whether message content is wrapped to viewport width
+	messageCache   []string // cache of syntax highlighted and wrapped message content
+	messageOffsets []int
 
 	// ui elements
 	content     viewport.Model