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/approval_test.go

@@ -5,7 +5,6 @@ import (
 	"strings"
 	"testing"
 
-	tea "github.com/charmbracelet/bubbletea"
 	"github.com/correx/tui-go/internal/protocol"
 	"github.com/correx/tui-go/internal/ws"
 )
@@ -40,12 +39,12 @@ func apprReq(reqID, tier string) protocol.ServerMessage {
 }
 
 // applyKey drives a key through the production handleKey entry and returns the model.
-func applyKey(m Model, k tea.KeyMsg) Model {
+func applyKey(m Model, k keyMsg) Model {
 	updated, _ := m.handleKey(k)
 	return updated.(Model)
 }
 
-func runeKey(r rune) tea.KeyMsg { return tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune{r}} }
+func runeKey(r rune) keyMsg { return keyMsg{Type: keyRunes, Runes: []rune{r}} }
 
 // decodeApproval pulls the single ApprovalResponse frame off the client and returns
 // its request id + decision. Fails if there isn't exactly one decision frame.
@@ -146,7 +145,7 @@ func TestApprovalHighTierArmCancelledByNavKey(t *testing.T) {
 	// no decision may be emitted.
 	m, client := approvalModel(apprReq("req-1", "T3"), apprReq("req-2", "T3"))
 	m = applyKey(m, runeKey('y')) // arm req-1
-	m = applyKey(m, tea.KeyMsg{Type: tea.KeyDown})
+	m = applyKey(m, keyMsg{Type: keyDown})
 	if m.approvalArmed {
 		t.Fatalf("nav key should disarm")
 	}
@@ -168,7 +167,7 @@ func TestApprovalQueueNavigatesAndCounts(t *testing.T) {
 		t.Fatalf("band should show queue count 1/3:\n%s", out)
 	}
 	// ↓ advances the selection; Pending follows the cursor.
-	m = applyKey(m, tea.KeyMsg{Type: tea.KeyDown})
+	m = applyKey(m, keyMsg{Type: keyDown})
 	s = m.session("s1")
 	if s.PendingIdx != 1 || s.Pending.RequestID != "req-2" {
 		t.Fatalf("↓ should select req-2, got idx=%d id=%s", s.PendingIdx, s.Pending.RequestID)
@@ -178,7 +177,7 @@ func TestApprovalQueueNavigatesAndCounts(t *testing.T) {
 		t.Fatalf("band should show 2/3 after ↓:\n%s", out)
 	}
 	// ↑ wraps back.
-	m = applyKey(m, tea.KeyMsg{Type: tea.KeyUp})
+	m = applyKey(m, keyMsg{Type: keyUp})
 	if m.session("s1").PendingIdx != 0 {
 		t.Fatalf("↑ should return to index 0")
 	}
@@ -238,7 +237,7 @@ func TestApprovalRejectAndSteerRoute(t *testing.T) {
 	for _, r := range "use sudo" {
 		m2 = applyKey(m2, runeKey(r))
 	}
-	m2 = applyKey(m2, tea.KeyMsg{Type: tea.KeyEnter})
+	m2 = applyKey(m2, keyMsg{Type: keyEnter})
 	frames := client2.Drain()
 	var note string
 	for _, f := range frames {