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/ideacard_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"
 )
@@ -43,7 +42,7 @@ func TestIdeaRemoveSendsDiscardAndDropsRow(t *testing.T) {
 	m, client := ideaModel()
 	_ = client.Drain() // drop the ListIdeas frame from openIdeas
 
-	updated, _ := m.handleOverlayKey(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("x")})
+	updated, _ := m.handleOverlayKey(keyMsg{Type: keyRunes, Runes: []rune("x")})
 	m = updated.(Model)
 
 	if len(m.ideas) != 1 || m.ideas[0].IdeaID != "i2" {