feat: add directory picker for OPDS downloads with per-server default path

When downloading a book via OPDS, a directory picker now lets the user
choose the save location instead of always saving to the SD root. Each
OPDS server has a configurable default download path (persisted in
opds.json) that the picker opens to. Falls back to "/" if the saved
path no longer exists on disk.

- Add DirectoryPickerActivity (browse-only directory view with "Save Here")
- Add PICKING_DIRECTORY state to OpdsBookBrowserActivity
- Add downloadPath field to OpdsServer with JSON serialization
- Add Download Path setting to OPDS server edit screen
- Extract sortFileList() to StringUtils for shared use
- Add i18n strings: STR_SAVE_HERE, STR_SELECT_FOLDER, STR_DOWNLOAD_PATH

Made-with: Cursor

src/util/StringUtils.cpp

@@ -1,5 +1,6 @@
 #include "StringUtils.h"
 
+#include <algorithm>
 #include <cstring>
 
 namespace StringUtils {
@@ -61,4 +62,43 @@ bool checkFileExtension(const String& fileName, const char* extension) {
   return localFile.endsWith(localExtension);
 }
 
+void sortFileList(std::vector<std::string>& entries) {
+  std::sort(begin(entries), end(entries), [](const std::string& str1, const std::string& str2) {
+    bool isDir1 = str1.back() == '/';
+    bool isDir2 = str2.back() == '/';
+    if (isDir1 != isDir2) return isDir1;
+
+    const char* s1 = str1.c_str();
+    const char* s2 = str2.c_str();
+
+    while (*s1 && *s2) {
+      if (isdigit(*s1) && isdigit(*s2)) {
+        while (*s1 == '0') s1++;
+        while (*s2 == '0') s2++;
+
+        int len1 = 0, len2 = 0;
+        while (isdigit(s1[len1])) len1++;
+        while (isdigit(s2[len2])) len2++;
+
+        if (len1 != len2) return len1 < len2;
+
+        for (int i = 0; i < len1; i++) {
+          if (s1[i] != s2[i]) return s1[i] < s2[i];
+        }
+
+        s1 += len1;
+        s2 += len2;
+      } else {
+        char c1 = tolower(*s1);
+        char c2 = tolower(*s2);
+        if (c1 != c2) return c1 < c2;
+        s1++;
+        s2++;
+      }
+    }
+
+    return *s1 == '\0' && *s2 != '\0';
+  });
+}
+
 }  // namespace StringUtils

src/util/StringUtils.h

@@ -3,6 +3,7 @@
 #include <WString.h>
 
 #include <string>
+#include <vector>
 
 namespace StringUtils {
 
@@ -19,4 +20,10 @@ std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
 bool checkFileExtension(const std::string& fileName, const char* extension);
 bool checkFileExtension(const String& fileName, const char* extension);
 
+/**
+ * Sort a file/directory list with directories first, using case-insensitive natural sort.
+ * Directory entries are identified by a trailing '/'.
+ */
+void sortFileList(std::vector<std::string>& entries);
+
 }  // namespace StringUtils