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/overlays.go

@@ -2,10 +2,11 @@ package app
 
 import (
 	"fmt"
+	"image/color"
 	"sort"
 	"strings"
 
-	"github.com/charmbracelet/lipgloss"
+	"charm.land/lipgloss/v2"
 	"github.com/charmbracelet/x/ansi"
 )
 
@@ -21,8 +22,8 @@ import (
 func (m Model) center(modal string) string {
 	if m.lastBase == "" {
 		return lipgloss.Place(m.width, m.height, lipgloss.Center, lipgloss.Center, modal,
-			lipgloss.WithWhitespaceBackground(m.theme.P.BgDeep),
-			lipgloss.WithWhitespaceForeground(m.theme.P.BgDeep))
+			lipgloss.WithWhitespaceStyle(lipgloss.NewStyle().
+				Background(m.theme.P.BgDeep).Foreground(m.theme.P.BgDeep)))
 	}
 	if lipgloss.Height(modal) >= m.height && lipgloss.Width(modal) >= m.width {
 		return modal // already a full-screen composite; don't re-dim it
@@ -143,7 +144,7 @@ func (m Model) titleLine(text string) string {
 	return lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Bold(true).Render(text)
 }
 
-func mbg(t Theme, s string, fg lipgloss.Color) string {
+func mbg(t Theme, s string, fg color.Color) string {
 	return lipgloss.NewStyle().Foreground(fg).Background(t.P.BgPanel).Render(s)
 }
 
@@ -1217,7 +1218,7 @@ func shortID(id string) string {
 	return id[:13] + "…"
 }
 
-func tierColor(t Theme, tier int) lipgloss.Color {
+func tierColor(t Theme, tier int) color.Color {
 	switch {
 	case tier >= 3:
 		return t.P.Bad