perf: Add stable keys to Lazy list items for performance (#323)

Added stable `key` parameters to `items(...)` and `itemsIndexed(...)` calls across 13 screens. This prevents Compose from treating all list mutations as full item replacements and eliminates unnecessary full list recompositions during state updates.

Modifications applied to:
- KeysScreen.kt, SharedComponents.kt, AchievementsScreen.kt, PluginsScreen.kt
- SessionsScreen.kt, CronJobsScreen.kt, McpServersScreen.kt, PairingScreen.kt
- ToolsetsScreen.kt, ChatScreen.kt, ModelScreen.kt, SkillsScreen.kt
- KanbanScreen.kt, ProfilesScreen.kt, WebhooksScreen.kt

Note: LogsScreen.kt was left without a key because logs are string lines that can be duplicated, and using `.hashCode()` would cause a "Key was already used" crash. Positional indexing is safer there.

Fixes #279

app/src/main/java/com/m57/hermescontrol/ui/pairing/PairingScreen.kt

@@ -121,7 +121,7 @@ fun PairingScreen(
                                     }
                                 }
 
-                                items(pending) { item ->
+                                items(pending, key = { "${it.platform}_${it.code ?: it.user_id}" }) { item ->
                                     Card(
                                         modifier = Modifier.fillMaxWidth(),
                                         colors =
@@ -201,7 +201,7 @@ fun PairingScreen(
                                     }
                                 }
                             } else {
-                                items(approved) { item ->
+                                items(approved, key = { "${it.platform}_${it.user_id}" }) { item ->
                                     Card(
                                         modifier = Modifier.fillMaxWidth(),
                                     ) {