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/slash_test.go

@@ -2,8 +2,6 @@ package app
 
 import (
 	"testing"
-
-	tea "github.com/charmbracelet/bubbletea"
 )
 
 // `/` as the first char of a chat line opens the command palette inline (opencode-style),
@@ -14,7 +12,7 @@ func TestSlashOpensPaletteOnEmptyLine(t *testing.T) {
 	m.inputMode = ModeRouter
 	m.inputBuffer = ""
 
-	next, _ := m.handleInsertKey(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("/")})
+	next, _ := m.handleInsertKey(keyMsg{Type: keyRunes, Runes: []rune("/")})
 	nm := next.(Model)
 
 	if nm.overlay != OverlayPalette {
@@ -33,7 +31,7 @@ func TestSlashLiteralMidLine(t *testing.T) {
 	m.inputBuffer = "path"
 	m.inputCursor = len("path")
 
-	next, _ := m.handleInsertKey(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("/")})
+	next, _ := m.handleInsertKey(keyMsg{Type: keyRunes, Runes: []rune("/")})
 	nm := next.(Model)
 
 	if nm.overlay != OverlayNone {