feat: Material You design system rewrite — Phases 1–4

Complete UI/UX rewrite of hermes-mobile with Material You design system.

Phase 1: Color/Theme/Type/Shapes/Spacing/Motion tokens
Phase 2: HermesScaffold, StateViews (skeleton/empty/error), SearchBar, StatusBadge, ToggleRow
Phase 3: Bottom nav + drawer navigation with NavigationController
Phase 4: All 17 screens rewritten with HermesScaffold and Material You patterns

Closes PR #3.

app/src/main/java/com/m57/hermescontrol/NavigationController.kt

@@ -3,14 +3,35 @@ package com.m57.hermescontrol
 import androidx.navigation3.runtime.NavBackStack
 import androidx.navigation3.runtime.NavKey
 
+/**
+ * Central navigation controller with deduplication guard.
+ *
+ * Primary screens (bottom nav tabs) clear the back stack and become the new
+ * root — this matches Material's bottom-nav pattern where each tab has its own
+ * back stack history (simplified: we just clear).
+ *
+ * B7 (Jun 18 2026): Never call `backStack.add()` directly from UI callbacks.
+ * Always route through [navigateTo] to prevent stacking duplicate screen
+ * entries that compete for touch events.
+ */
 object NavigationController {
     var backStack: NavBackStack<NavKey>? = null
 
+    // Bottom-nav primary screens — tapping one clears the stack to root.
+    private val primaryScreens: Set<NavKey> =
+        setOf(
+            ChatScreen,
+            SkillsScreen,
+            CronJobsScreen,
+            SystemScreen,
+            SettingsScreen,
+        )
+
     fun navigateTo(key: NavKey) {
         val stack = backStack ?: return
         if (stack.lastOrNull() == key) return
 
-        if (key == ChatScreen || key == SkillsScreen || key == CronJobsScreen) {
+        if (key in primaryScreens) {
             stack.clear()
         }
         stack.add(key)