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/clarcard_test.go

@@ -4,7 +4,6 @@ import (
 	"strings"
 	"testing"
 
-	tea "github.com/charmbracelet/bubbletea"
 	"github.com/correx/tui-go/internal/protocol"
 	"github.com/correx/tui-go/internal/ws"
 )
@@ -43,13 +42,13 @@ func TestClarificationEntersStateAndRenders(t *testing.T) {
 func TestClarificationSelectAndSubmit(t *testing.T) {
 	m := clarModel()
 	// move the option cursor to "Vue" (index 1) and select it
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyRight})
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeySpace})
+	m = applyClarKey(m, keyMsg{Type: keyRight})
+	m = applyClarKey(m, keyMsg{Type: keySpace})
 	if !m.clarChosenAt(0, 1) {
 		t.Fatalf("expected option 1 chosen, chosen=%v", m.clarChosen)
 	}
 	// submit
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyEnter})
+	m = applyClarKey(m, keyMsg{Type: keyEnter})
 	if s := m.session("s1"); s == nil || s.Clar != nil {
 		t.Fatalf("expected clarification cleared after submit")
 	}
@@ -68,8 +67,8 @@ func TestClarificationSubmitGuardWaitsForAllAnswers(t *testing.T) {
 		},
 	})
 	// answer only the first question, then attempt submit
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeySpace})
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyEnter})
+	m = applyClarKey(m, keyMsg{Type: keySpace})
+	m = applyClarKey(m, keyMsg{Type: keyEnter})
 	if s := m.session("s1"); s == nil || s.Clar == nil {
 		t.Fatalf("expected clarification to remain with partial answers")
 	}
@@ -80,14 +79,14 @@ func TestClarificationSubmitGuardWaitsForAllAnswers(t *testing.T) {
 
 func TestClarificationCustomTextAnswer(t *testing.T) {
 	m := clarModel()
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("e")})
+	m = applyClarKey(m, keyMsg{Type: keyRunes, Runes: []rune("e")})
 	if !m.clarTyping {
 		t.Fatalf("expected typing mode after 'e'")
 	}
 	for _, r := range "Solid" {
-		m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune{r}})
+		m = applyClarKey(m, keyMsg{Type: keyRunes, Runes: []rune{r}})
 	}
-	m = applyClarKey(m, tea.KeyMsg{Type: tea.KeyEnter})
+	m = applyClarKey(m, keyMsg{Type: keyEnter})
 	if m.clarTyping {
 		t.Fatalf("expected typing committed")
 	}
@@ -99,7 +98,7 @@ func TestClarificationCustomTextAnswer(t *testing.T) {
 	}
 }
 
-func applyClarKey(m Model, k tea.KeyMsg) Model {
+func applyClarKey(m Model, k keyMsg) Model {
 	updated, _ := m.handleClarificationKey(k)
 	return updated.(Model)
 }