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/proposecard.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"
 )
 
@@ -24,7 +24,7 @@ func (m *Model) proposeInitState() {
 
 // --- key handling ---
 
-func (m Model) handleProposeKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
+func (m Model) handleProposeKey(k keyMsg) (tea.Model, tea.Cmd) {
 	s := m.session(m.selectedID)
 	if s == nil || s.Propose == nil {
 		return m, nil
@@ -34,15 +34,15 @@ func (m Model) handleProposeKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 	}
 	n := len(s.Propose.Candidates) // index n == the custom slot
 	switch k.Type {
-	case tea.KeyEsc:
+	case keyEsc:
 		m.proposeDismissed = true
-	case tea.KeyUp:
+	case keyUp:
 		m.proposeCursor = clampInt(m.proposeCursor-1, 0, n)
-	case tea.KeyDown:
+	case keyDown:
 		m.proposeCursor = clampInt(m.proposeCursor+1, 0, n)
-	case tea.KeyEnter, tea.KeySpace:
+	case keyEnter, keySpace:
 		return m.proposeSubmit()
-	case tea.KeyRunes:
+	case keyRunes:
 		switch string(k.Runes) {
 		case "k":
 			m.proposeCursor = clampInt(m.proposeCursor-1, 0, n)
@@ -56,17 +56,17 @@ func (m Model) handleProposeKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 	return m, nil
 }
 
-func (m Model) handleProposeTyping(k tea.KeyMsg) (tea.Model, tea.Cmd) {
+func (m Model) handleProposeTyping(k keyMsg) (tea.Model, tea.Cmd) {
 	switch k.Type {
-	case tea.KeyEsc:
+	case keyEsc:
 		m.proposeTyping = false
-	case tea.KeyEnter:
+	case keyEnter:
 		return m.proposeSubmit() // commit the custom answer and send it as chat
-	case tea.KeyBackspace:
+	case keyBackspace:
 		if len(m.proposeText) > 0 {
 			m.proposeText = m.proposeText[:len(m.proposeText)-1]
 		}
-	case tea.KeyRunes, tea.KeySpace:
+	case keyRunes, keySpace:
 		m.proposeText += string(k.Runes)
 	}
 	return m, nil