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/theme.go

@@ -1,36 +1,40 @@
 package app
 
-import "github.com/charmbracelet/lipgloss"
+import (
+	"image/color"
+
+	"charm.land/lipgloss/v2"
+)
 
 // Palette is the "Soft" chrome from docs/visual/ref with the blue accent:
 // rounded borders, opaque dark panels, generous spacing.
 type Palette struct {
-	Bg       lipgloss.Color // panel/background fill
-	BgDeep   lipgloss.Color // outermost backdrop (slightly darker)
-	BgPanel  lipgloss.Color // inside-panel fill
-	Sel      lipgloss.Color // selection row background
-	Border   lipgloss.Color // idle panel border
-	BorderHi lipgloss.Color // active panel border
-	Fg       lipgloss.Color // primary text
-	FgStrong lipgloss.Color // headings / emphasis
-	Dim      lipgloss.Color // labels, secondary text
-	Faint    lipgloss.Color // tertiary, hints
-	Accent   lipgloss.Color // blue accent
-	Accent2  lipgloss.Color // secondary accent
+	Bg       color.Color // panel/background fill
+	BgDeep   color.Color // outermost backdrop (slightly darker)
+	BgPanel  color.Color // inside-panel fill
+	Sel      color.Color // selection row background
+	Border   color.Color // idle panel border
+	BorderHi color.Color // active panel border
+	Fg       color.Color // primary text
+	FgStrong color.Color // headings / emphasis
+	Dim      color.Color // labels, secondary text
+	Faint    color.Color // tertiary, hints
+	Accent   color.Color // blue accent
+	Accent2  color.Color // secondary accent
 
-	OK   lipgloss.Color
-	Warn lipgloss.Color
-	Bad  lipgloss.Color
+	OK   color.Color
+	Warn color.Color
+	Bad  color.Color
 
-	DiffAdd lipgloss.Color // added-line cell background
-	DiffDel lipgloss.Color // removed-line cell background
+	DiffAdd color.Color // added-line cell background
+	DiffDel color.Color // removed-line cell background
 
-	CatLifecycle lipgloss.Color
-	CatContext   lipgloss.Color
-	CatInference lipgloss.Color
-	CatTool      lipgloss.Color
-	CatDomain    lipgloss.Color
-	CatApproval  lipgloss.Color
+	CatLifecycle color.Color
+	CatContext   color.Color
+	CatInference color.Color
+	CatTool      color.Color
+	CatDomain    color.Color
+	CatApproval  color.Color
 }
 
 // SoftBlue mirrors the reference Soft theme (#14161a bg, #ced3da fg) with the
@@ -91,7 +95,7 @@ type Theme struct {
 // NewTheme builds the style set for a palette.
 func NewTheme(p Palette) Theme {
 	base := lipgloss.NewStyle().Background(p.Bg).Foreground(p.Fg)
-	roundBase := func(border lipgloss.Color) lipgloss.Style {
+	roundBase := func(border color.Color) lipgloss.Style {
 		return lipgloss.NewStyle().
 			Border(lipgloss.RoundedBorder()).
 			BorderForeground(border).
@@ -128,7 +132,7 @@ func NewTheme(p Palette) Theme {
 }
 
 // categoryColor maps an event category label to its accent color.
-func (t Theme) categoryColor(cat string) lipgloss.Color {
+func (t Theme) categoryColor(cat string) color.Color {
 	switch cat {
 	case "Lifecycle":
 		return t.P.CatLifecycle