fix: back navigation from BMPViewer (#1597)

## Summary

This fixes navigating back from the BMP Viewer to the FileBrowser which
was broken when moving to the new ActivityManager

This is fixed by making FileBrowserActivity able to take a full file
path on enter and splitting the basePath and fileName from it and
navigating to the correct place.

fixes:
https://github.com/crosspoint-reader/crosspoint-reader/issues/1553

duplicates:
https://github.com/crosspoint-reader/crosspoint-reader/pull/910 to some
extend but mine has the file cursor at the correct file instead of the
first one in the folder

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**PARTIALLY**_

---------

Co-authored-by: Jan Ivanov <jan.ivanov@sirma.com>

src/activities/home/FileBrowserActivity.cpp

@@ -108,9 +108,28 @@ void FileBrowserActivity::loadFiles() {
 void FileBrowserActivity::onEnter() {
   Activity::onEnter();
 
-  loadFiles();
   selectorIndex = 0;
 
+  auto root = Storage.open(basepath.c_str());
+  if (!root) {
+    basepath = "/";
+    loadFiles();
+  } else if (!root.isDirectory()) {
+    root.close();
+    lockLongPressBack = mappedInput.isPressed(MappedInputManager::Button::Back);
+
+    const std::string oldPath = basepath;
+    basepath = FsHelpers::extractFolderPath(basepath);
+    loadFiles();
+
+    const auto pos = oldPath.find_last_of('/');
+    const std::string fileName = oldPath.substr(pos + 1);
+    selectorIndex = findEntry(fileName);
+  } else {
+    root.close();
+    loadFiles();
+  }
+
   requestUpdate();
 }
 
@@ -129,11 +148,19 @@ void FileBrowserActivity::clearFileMetadata(const std::string& fullPath) {
 
 void FileBrowserActivity::loop() {
   // Long press BACK (1s+) goes to root folder
+  // but Long press BACK (1s+) from ReaderActivity sends us here with the MappedInput already set.
+  // So ignore it the first time.
   if (mappedInput.isPressed(MappedInputManager::Button::Back) && mappedInput.getHeldTime() >= GO_HOME_MS &&
-      basepath != "/") {
+      basepath != "/" && !lockLongPressBack) {
     basepath = "/";
     loadFiles();
     selectorIndex = 0;
+    requestUpdate();
+    return;
+  }
+
+  if (lockLongPressBack && mappedInput.wasReleased(MappedInputManager::Button::Back)) {
+    lockLongPressBack = false;
     return;
   }