fix: Init lastSleepImage (edge case) (#1360)

## Summary

* **What is the goal of this PR?** fix edge case for definition

## Additional Context

If loadFromFile() returns false (no state file exists — first boot, or
SD missing), lastSleepImage is never set and contains garbage.
[SleepActivity.cpp:83] then uses it in a while comparison to avoid
repeating the same image. The JSON path (doc["lastSleepImage"] |
(uint8_t)0) handles it, but only if the file exists.

---

### AI Usage

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Did you use AI tools to help write this code? _** NO **_

src/CrossPointState.cpp

@@ -63,7 +63,7 @@ bool CrossPointState::loadFromBinaryFile() {
   if (version >= 2) {
     serialization::readPod(inputFile, lastSleepImage);
   } else {
-    lastSleepImage = 0;
+    lastSleepImage = UINT8_MAX;
   }
 
   if (version >= 3) {

src/CrossPointState.h

@@ -1,4 +1,5 @@
 #pragma once
+#include <cstdint>
 #include <iosfwd>
 #include <string>
 
@@ -8,7 +9,7 @@ class CrossPointState {
 
  public:
   std::string openEpubPath;
-  uint8_t lastSleepImage;
+  uint8_t lastSleepImage = UINT8_MAX;  // UINT8_MAX = unset sentinel
   uint8_t readerActivityLoadCount = 0;
   bool lastSleepFromReader = false;
   ~CrossPointState() = default;

src/JsonSettingsIO.cpp

@@ -87,7 +87,7 @@ bool JsonSettingsIO::loadState(CrossPointState& s, const char* json) {
   }
 
   s.openEpubPath = doc["openEpubPath"] | std::string("");
-  s.lastSleepImage = doc["lastSleepImage"] | (uint8_t)0;
+  s.lastSleepImage = doc["lastSleepImage"] | (uint8_t)UINT8_MAX;
   s.readerActivityLoadCount = doc["readerActivityLoadCount"] | (uint8_t)0;
   s.lastSleepFromReader = doc["lastSleepFromReader"] | false;
   return true;

src/activities/boot_sleep/SleepActivity.cpp

@@ -80,7 +80,7 @@ void SleepActivity::renderCustomSleepScreen() const {
       // Generate a random number between 1 and numFiles
       auto randomFileIndex = random(numFiles);
       // If we picked the same image as last time, reroll
-      while (numFiles > 1 && randomFileIndex == APP_STATE.lastSleepImage) {
+      while (numFiles > 1 && APP_STATE.lastSleepImage != UINT8_MAX && randomFileIndex == APP_STATE.lastSleepImage) {
         randomFileIndex = random(numFiles);
       }
       APP_STATE.lastSleepImage = randomFileIndex;