tui: migrate Bubble Tea v1 → v2 (charm.land), enable Shift+Enter

Move the Go TUI from github.com/charmbracelet/{bubbletea,lipgloss} to the v2
ecosystem at charm.land/{bubbletea,lipgloss}/v2 (Go 1.25). v2's kitty keyboard
disambiguation is on by default, so Shift+Enter now reliably inserts a newline
in the composer (Ctrl+J / Alt+Enter kept as fallbacks for non-kitty terminals).

Approach: rather than rewrite ~190 key-match sites to v2's (Code, Mod) idiom, a
small shim (internal/app/key.go) converts a v2 KeyPressMsg into the v1-shaped
keyMsg the handlers already expect, at the single Update boundary. The rest is
mechanical:
- key constants tea.Key* → shim consts; tea.KeyMsg → keyMsg.
- lipgloss.Color is now a func returning color.Color, not a type → fields/params
  retyped to image/color.Color (theme/diff/overlays/view).
- v2 View() returns tea.View: render() builds the string, View() wraps it and
  carries AltScreen (alt-screen is a per-frame View field now, not a program opt).
- WithWhitespaceBackground/Foreground → WithWhitespaceStyle(Style).
- preview: drop lipgloss.SetColorProfile (v2 renders truecolor by default).

Build, vet, gofmt, and the full test suite are green; preview renders truecolor
across kinds.

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

apps/tui-go/main.go

@@ -8,7 +8,7 @@ import (
 	"fmt"
 	"os"
 
-	tea "github.com/charmbracelet/bubbletea"
+	tea "charm.land/bubbletea/v2"
 	"github.com/correx/tui-go/internal/app"
 	"github.com/correx/tui-go/internal/ws"
 )
@@ -33,7 +33,8 @@ func main() {
 	defer cancel()
 	go client.Run(ctx)
 
-	p := tea.NewProgram(app.NewModel(client), tea.WithAltScreen())
+	// Alt-screen is requested per-frame via the View's AltScreen field in v2.
+	p := tea.NewProgram(app.NewModel(client))
 	if _, err := p.Run(); err != nil {
 		fmt.Fprintln(os.Stderr, "correx tui error:", err)
 		os.Exit(1)