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/view.go

@@ -2,11 +2,13 @@ package app
 
 import (
 	"fmt"
+	"image/color"
 	"os"
 	"strings"
 	"time"
 
-	"github.com/charmbracelet/lipgloss"
+	tea "charm.land/bubbletea/v2"
+	"charm.land/lipgloss/v2"
 	"github.com/charmbracelet/x/ansi"
 )
 
@@ -28,8 +30,15 @@ const (
 // and full-width chrome.
 func (m Model) narrow() bool { return m.width < narrowWidth }
 
-// View renders the whole frame purely from the model (immediate-mode).
-func (m Model) View() string {
+// View renders the whole frame purely from the model (immediate-mode). In
+// bubbletea v2 the View carries terminal state (alt-screen, keyboard mode), so
+// render() builds the string and View() wraps it with the screen settings.
+func (m Model) View() tea.View {
+	return tea.View{Content: m.render(), AltScreen: true}
+}
+
+// render builds the frame string.
+func (m Model) render() string {
 	if m.quitting {
 		return ""
 	}
@@ -79,7 +88,7 @@ func (m Model) View() string {
 func (m Model) renderStatus() string {
 	t := m.theme
 	bg := t.P.BgPanel
-	span := func(s string, fg lipgloss.Color) string {
+	span := func(s string, fg color.Color) string {
 		return lipgloss.NewStyle().Foreground(fg).Background(bg).Render(s)
 	}
 	nrw := m.narrow()
@@ -376,12 +385,12 @@ func (m Model) renderLauncher(w, h int) string {
 		inW = 24
 	}
 	left := lipgloss.Place(leftW, h, lipgloss.Center, lipgloss.Center, m.launcherInput(inW),
-		lipgloss.WithWhitespaceBackground(t.P.Bg))
+		lipgloss.WithWhitespaceStyle(lipgloss.NewStyle().Background(t.P.Bg)))
 	if railW == 0 {
 		return left
 	}
 	rail := lipgloss.Place(railW, h, lipgloss.Left, lipgloss.Top, m.launcherRail(railW),
-		lipgloss.WithWhitespaceBackground(t.P.Bg))
+		lipgloss.WithWhitespaceStyle(lipgloss.NewStyle().Background(t.P.Bg)))
 	return lipgloss.JoinHorizontal(lipgloss.Top, left, rail)
 }
 
@@ -738,7 +747,7 @@ func (m Model) eventStreamBadge() string {
 		return t.span("○ idle", t.P.Faint)
 	}
 	if s := m.session(m.selectedID); s != nil {
-		badge := func(fg lipgloss.Color, text string) string {
+		badge := func(fg color.Color, text string) string {
 			return lipgloss.NewStyle().Foreground(fg).Background(t.P.Bg).Render(text)
 		}
 		switch u := strings.ToUpper(s.Status); {
@@ -764,7 +773,7 @@ func shortPath(p string) string {
 }
 
 // fill left-justifies a styled line and pads with bg to width w.
-func (m Model) fill(s string, w int, bg lipgloss.Color) string {
+func (m Model) fill(s string, w int, bg color.Color) string {
 	return " " + padTo(s, w-1, bg)
 }
 
@@ -772,7 +781,7 @@ func (m Model) fill(s string, w int, bg lipgloss.Color) string {
 // so the line never overflows w. The left segment is shrunk first (the right holds
 // the high-signal status — connection clock / active spinner); the right is clamped
 // only as a last resort. ANSI-aware so truncation can't slice through escape codes.
-func (m Model) justify(left, right string, w int, bg lipgloss.Color) string {
+func (m Model) justify(left, right string, w int, bg color.Color) string {
 	avail := w - 2 // leading + trailing space
 	lw := lipgloss.Width(left)
 	rw := lipgloss.Width(right)
@@ -798,7 +807,7 @@ func (m Model) justify(left, right string, w int, bg lipgloss.Color) string {
 
 // padTo pads (or truncates) a possibly-styled string to visible width w, filling
 // with the given background so the panel stays opaque.
-func padTo(s string, w int, bg lipgloss.Color) string {
+func padTo(s string, w int, bg color.Color) string {
 	vw := lipgloss.Width(s)
 	if vw == w {
 		return s
@@ -825,7 +834,7 @@ func padRaw(s string, w int) string {
 	return s + strings.Repeat(" ", w-n)
 }
 
-func statusColor(t Theme, status string) lipgloss.Color {
+func statusColor(t Theme, status string) color.Color {
 	u := strings.ToUpper(status)
 	switch {
 	case strings.Contains(u, "FAIL"):
@@ -862,7 +871,7 @@ func statusGlyph(t Theme, status string) string {
 }
 
 // span renders text with foreground fg over the panel background.
-func (t Theme) span(s string, fg lipgloss.Color) string {
+func (t Theme) span(s string, fg color.Color) string {
 	return lipgloss.NewStyle().Foreground(fg).Background(t.P.Bg).Render(s)
 }