fix(ux): keep Settings highlighted in bottom nav on drill-down sub-pages (issue #637) (#643)

* fix(ux): keep Settings highlighted in bottom nav on drill-down sub-pages (issue #637)

Map Settings drill-down destinations (SettingsConnection, SettingsAppearance, SettingsChat, SettingsNavBar, SettingsBehavior, SettingsAbout) back to SettingsScreen for bottom-nav selection only - navigation is unaffected.

- Add selectedBottomNavDestination() helper in Navigation.kt
- Extract bar-height logic into Modifier.bottomNavigationHeight() (ICON_AND_TEXT now uses heightIn(min=80.dp) so the standard bar can grow)
- BottomNavigationSelectionTest (JVM): selection mapping logic
- BottomNavigationLayoutTest (androidTest): per-mode bar height assertions

* docs(navigation): warn that new Settings sub-pages must be added to settingsDestinations

Addresses agy review (PR #643): documents the fragility of the hardcoded settingsDestinations set so future Settings drill-down screens don't silently drop the bottom-nav highlight. The sealed-class/marker-interface refactor was declined as scope creep (issue #637 explicitly scopes Settings-only; keys are flat data objects).

* fix: language selector

---------

Co-authored-by: M57 <Hy4ri@users.noreply.github.com>

app/src/test/java/com/m57/hermescontrol/BottomNavigationSelectionTest.kt

@@ -0,0 +1,59 @@
+package com.m57.hermescontrol
+
+import androidx.navigation3.runtime.NavKey
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+/**
+ * Verifies that the bottom-nav selection key maps Settings drill-down
+ * destinations back to [SettingsScreen] so the indicator stays highlighted,
+ * while every other screen is returned unchanged.
+ */
+class BottomNavigationSelectionTest {
+    @Test
+    fun `SettingsScreen selects itself`() {
+        assertEquals(SettingsScreen, selectedBottomNavDestination(SettingsScreen))
+    }
+
+    @Test
+    fun `every Settings drill-down maps back to SettingsScreen`() {
+        val drillDowns: List<NavKey> =
+            listOf(
+                SettingsConnection,
+                SettingsAppearance,
+                SettingsChat,
+                SettingsNavBar,
+                SettingsBehavior,
+                SettingsAbout,
+            )
+        for (destination in drillDowns) {
+            assertEquals(
+                "Expected $destination to map to SettingsScreen",
+                SettingsScreen,
+                selectedBottomNavDestination(destination),
+            )
+        }
+    }
+
+    @Test
+    fun `non-Settings screens are returned unchanged`() {
+        val others: List<NavKey> =
+            listOf(
+                ChatScreen,
+                CronJobsScreen,
+                WebhooksScreen,
+                GatewayScreen,
+                SkillsScreen,
+                LandingScreen,
+                AuthLoginScreen,
+                PairingCodeEntryScreen,
+            )
+        for (screen in others) {
+            assertEquals(
+                "Expected $screen to be returned unchanged",
+                screen,
+                selectedBottomNavDestination(screen),
+            )
+        }
+    }
+}