Refactor map screen panel state to nullable contracts.

This replaces visibility booleans and error state fields with nullable `searchPanelState`/`devicePanelState` and action-based error toasts for one-shot UI effects.

Made-with: Cursor

.cursor/rules/screen-ui-contract.mdc

@@ -0,0 +1,41 @@
+---
+description: Screen UI contract structure for State, Action, and Event
+alwaysApply: false
+---
+# Screen UI Contract Rule
+
+Every screen must define its contract in a `*ScreenContract.kt` file using `State`, `Action`, and `Event`.
+
+## Required structure
+
+1. Define `State` with all static UI data required to render the screen.
+2. Define `Action` (enum or sealed interface) for what the ViewModel does.
+3. Define `Event` (enum or sealed interface) for user interactions.
+
+## State guidelines
+
+- `State` must include stable screen data (for example `deviceName`, `deviceId`, `isSharingEnabled`).
+- Do not put user interaction triggers in `State`; those belong to `Event`.
+
+## Action guidelines
+
+- Use `Action` for ViewModel-driven outcomes such as navigation or side effects.
+- Example: `Action.NavigateTo*Screen`, `Action.Show*Toast`
+
+## Event guidelines
+
+- Use `Event` for user-originated input.
+- Examples: `Event.*ButtonClicked`, `Event.*InputChanged(value)`.
+
+## Example
+
+```kotlin
+data class State(val deviceName: String, val deviceId: String, val isSharingEnabled: Boolean)
+
+sealed interface Action { data object NavigateToScreenX : Action }
+
+sealed interface Event {
+    data object XButtonClicked : Event
+    data class InputChanged(val value: String) : Event
+}
+```