feat(tui): dock approval gate + two-column diff viewer

Replace the floating approval modal (which blanked the whole UI) with an
opencode-style band that takes the input bar's slot while a gate is
pending: tool/tier/risk header above a side-by-side old|new diff, with
the session output and event stream still visible above it. The band
height adapts to the diff length.

Add a unified-diff parser that aligns removals/additions into split rows
(blank left cell for a create, blank right for a delete) and a
two-column renderer shared by the band preview and the ^x fullscreen
view. Wire plain a/r to approve/reject to match the advertised keys.

apps/tui-go/internal/app/theme.go

@@ -5,23 +5,26 @@ import "github.com/charmbracelet/lipgloss"
 // 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       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
 
 	OK   lipgloss.Color
 	Warn lipgloss.Color
 	Bad  lipgloss.Color
 
+	DiffAdd lipgloss.Color // added-line cell background
+	DiffDel lipgloss.Color // removed-line cell background
+
 	CatLifecycle lipgloss.Color
 	CatContext   lipgloss.Color
 	CatInference lipgloss.Color
@@ -50,6 +53,9 @@ var SoftBlue = Palette{
 	Warn: lipgloss.Color("#e8c06a"),
 	Bad:  lipgloss.Color("#e87f7f"),
 
+	DiffAdd: lipgloss.Color("#1b2a1d"),
+	DiffDel: lipgloss.Color("#2c1c1f"),
+
 	CatLifecycle: lipgloss.Color("#c98fd9"),
 	CatContext:   lipgloss.Color("#b9c0c9"),
 	CatInference: lipgloss.Color("#56c8d8"),
@@ -69,17 +75,17 @@ type Theme struct {
 	PanelTitle lipgloss.Style // uppercase dim header label
 	TitleHi    lipgloss.Style // active header label
 
-	Text     lipgloss.Style
-	Strong   lipgloss.Style
-	Dim      lipgloss.Style
-	Faint    lipgloss.Style
-	Accent   lipgloss.Style
-	SelRow   lipgloss.Style
-	Status   lipgloss.Style // top bar
-	Footer   lipgloss.Style // bottom hints
-	Input    lipgloss.Style // input bar panel
-	Overlay  lipgloss.Style // modal box
-	Scrim    lipgloss.Style // dim backdrop behind modals
+	Text    lipgloss.Style
+	Strong  lipgloss.Style
+	Dim     lipgloss.Style
+	Faint   lipgloss.Style
+	Accent  lipgloss.Style
+	SelRow  lipgloss.Style
+	Status  lipgloss.Style // top bar
+	Footer  lipgloss.Style // bottom hints
+	Input   lipgloss.Style // input bar panel
+	Overlay lipgloss.Style // modal box
+	Scrim   lipgloss.Style // dim backdrop behind modals
 }
 
 // NewTheme builds the style set for a palette.