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/internal/app/diff.go

@@ -1,10 +1,11 @@
 package app
 
 import (
+	"image/color"
 	"strconv"
 	"strings"
 
-	"github.com/charmbracelet/lipgloss"
+	"charm.land/lipgloss/v2"
 )
 
 type diffKind int
@@ -196,7 +197,7 @@ func (m Model) diffCell(num int, text string, kind diffKind, w int) string {
 	if kind == diffBlank {
 		return lipgloss.NewStyle().Background(t.P.Bg).Render(strings.Repeat(" ", w))
 	}
-	var bg, textFg, signFg lipgloss.Color
+	var bg, textFg, signFg color.Color
 	sign := " "
 	switch kind {
 	case diffAdd: