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/config_overlay.go

@@ -3,8 +3,8 @@ package app
 import (
 	"strings"
 
-	tea "github.com/charmbracelet/bubbletea"
-	"github.com/charmbracelet/lipgloss"
+	tea "charm.land/bubbletea/v2"
+	"charm.land/lipgloss/v2"
 	"github.com/correx/tui-go/internal/protocol"
 )
 
@@ -23,40 +23,40 @@ func (m *Model) openConfig() {
 
 // handleConfigKey owns every key while the config overlay is open. In edit mode it captures the
 // value being typed; otherwise it navigates fields, stages edits, and saves.
-func (m Model) handleConfigKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
+func (m Model) handleConfigKey(k keyMsg) (tea.Model, tea.Cmd) {
 	if m.configEditing {
 		switch k.Type {
-		case tea.KeyEsc:
+		case keyEsc:
 			m.configEditing = false
 			m.configEditBuf = ""
-		case tea.KeyEnter:
+		case keyEnter:
 			if m.configIndex >= 0 && m.configIndex < len(m.configFields) {
 				m.configStaged[m.configFields[m.configIndex].Key] = m.configEditBuf
 			}
 			m.configEditing = false
 			m.configEditBuf = ""
-		case tea.KeyBackspace:
+		case keyBackspace:
 			if n := len(m.configEditBuf); n > 0 {
 				m.configEditBuf = m.configEditBuf[:n-1]
 			}
-		case tea.KeyRunes, tea.KeySpace:
+		case keyRunes, keySpace:
 			m.configEditBuf += string(k.Runes)
 		}
 		return m, nil
 	}
 
 	switch {
-	case k.Type == tea.KeyEsc || runeIs(k, "g"):
+	case k.Type == keyEsc || runeIs(k, "g"):
 		m.overlay = OverlayNone
-	case k.Type == tea.KeyUp || runeIs(k, "k"):
+	case k.Type == keyUp || runeIs(k, "k"):
 		if m.configIndex > 0 {
 			m.configIndex--
 		}
-	case k.Type == tea.KeyDown || runeIs(k, "j"):
+	case k.Type == keyDown || runeIs(k, "j"):
 		if m.configIndex < len(m.configFields)-1 {
 			m.configIndex++
 		}
-	case k.Type == tea.KeyEnter || k.Type == tea.KeySpace:
+	case k.Type == keyEnter || k.Type == keySpace:
 		m = m.actOnConfigField()
 	case runeIs(k, "s"):
 		if len(m.configStaged) > 0 {