feat(tui-go): multi-line input — Ctrl+J / Alt+Enter newline, growing box (Phase 2)

Enter still submits; Ctrl+J (always) and Alt+Enter (most terminals) insert a
newline at the cursor. True Shift+Enter isn't distinguishable from Enter in
bubbletea v1.3.10 (terminals send the same byte, no kitty-protocol parsing), so
these are the working stand-ins — noted in the input hint.

- editorLines() renders the buffer as styled rows with the caret at the cursor,
  splitting on \n and capping at maxInputLines (last rows kept). Both the idle
  launcher input and the in-session bottom bar use it and grow to fit; View()
  sizes the bottom bar via inputBarHeight() instead of the fixed inputH.
- Footer gains a "^J newline" hint in insert mode (non-filter); the launcher
  sub-line shows "⌥↵/^J newline". New "compose" preview kind + editor_lines_test
  (split + height cap).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -22,6 +22,15 @@ func PreviewFrame(kind string, w, h int) string {
 		m.bgUpdates = 3
 		m.selectedID = "04a546aa"
 
+	case "compose":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.editMode = ModeInsert
+		m.inputMode = ModeRouter
+		m.inputBuffer = "fix the healthcheck script\nso it retries three times\nbefore it fails"
+		m.inputCursor = len(m.inputBuffer)
+
 	case "resume":
 		m.connected = true
 		m.currentModel = "llama-cpp:default"