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/update.go

@@ -440,7 +440,20 @@ func (m Model) handleInsertKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 			m.inputMode = ModeRouter
 		}
 	case tea.KeyEnter:
+		// Alt+Enter inserts a newline; a bare Enter submits. (Shift+Enter is not
+		// distinguishable from Enter in this terminal stack — see Ctrl+J below.)
+		if k.Alt && m.inputMode != ModeFilter {
+			m.appendRunes("\n")
+			return m, nil
+		}
 		return m.submit()
+	case tea.KeyCtrlJ:
+		// Reliable "newline without sending" (Ctrl+J is the literal LF key) for the
+		// single-line filter excluded.
+		if m.inputMode != ModeFilter {
+			m.appendRunes("\n")
+		}
+		return m, nil
 	case tea.KeyBackspace:
 		m.backspace()
 		m.syncFilter()