feat: add Screen interface and Context for UI architecture

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/screen.go

@@ -0,0 +1,11 @@
+package ui
+
+import tea "github.com/charmbracelet/bubbletea"
+
+// Screen represents an independent screen with its own Update and View logic.
+// Update returns the next Screen (can return itself or a different screen for transitions)
+// plus a tea.Cmd for async operations.
+type Screen interface {
+	Update(msg tea.Msg, ctx *Context) (Screen, tea.Cmd)
+	View(ctx *Context) string
+}