fix: OPDS browser OOM (#403)

## Summary

- Rewrite OpdsParser to stream parsing instead of full content
- Fix OOM due to big http xml response

Closes #385 

---

### 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? _**NO**_

src/activities/browser/OpdsBookBrowserActivity.cpp

@@ -3,6 +3,7 @@
 #include <Epub.h>
 #include <GfxRenderer.h>
 #include <HardwareSerial.h>
+#include <OpdsStream.h>
 #include <WiFi.h>
 
 #include "CrossPointSettings.h"
@@ -265,23 +266,27 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
   std::string url = UrlUtils::buildUrl(serverUrl, path);
   Serial.printf("[%lu] [OPDS] Fetching: %s\n", millis(), url.c_str());
 
-  std::string content;
-  if (!HttpDownloader::fetchUrl(url, content)) {
-    state = BrowserState::ERROR;
-    errorMessage = "Failed to fetch feed";
-    updateRequired = true;
-    return;
+  OpdsParser parser;
+
+  {
+    OpdsParserStream stream{parser};
+    if (!HttpDownloader::fetchUrl(url, stream)) {
+      state = BrowserState::ERROR;
+      errorMessage = "Failed to fetch feed";
+      updateRequired = true;
+      return;
+    }
   }
 
-  OpdsParser parser;
-  if (!parser.parse(content.c_str(), content.size())) {
+  if (!parser) {
     state = BrowserState::ERROR;
     errorMessage = "Failed to parse feed";
     updateRequired = true;
     return;
   }
 
-  entries = parser.getEntries();
+  entries = std::move(parser).getEntries();
+  Serial.printf("[%lu] [OPDS] Found %d entries\n", millis(), entries.size());
   selectorIndex = 0;
 
   if (entries.empty()) {

src/network/HttpDownloader.cpp

@@ -2,6 +2,7 @@
 
 #include <HTTPClient.h>
 #include <HardwareSerial.h>
+#include <StreamString.h>
 #include <WiFiClient.h>
 #include <WiFiClientSecure.h>
 
@@ -9,7 +10,7 @@
 
 #include "util/UrlUtils.h"
 
-bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
+bool HttpDownloader::fetchUrl(const std::string& url, Stream& outContent) {
   // Use WiFiClientSecure for HTTPS, regular WiFiClient for HTTP
   std::unique_ptr<WiFiClient> client;
   if (UrlUtils::isHttpsUrl(url)) {
@@ -34,10 +35,20 @@ bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
     return false;
   }
 
-  outContent = http.getString().c_str();
+  http.writeToStream(&outContent);
+
   http.end();
 
-  Serial.printf("[%lu] [HTTP] Fetched %zu bytes\n", millis(), outContent.size());
+  Serial.printf("[%lu] [HTTP] Fetch success\n", millis());
+  return true;
+}
+
+bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
+  StreamString stream;
+  if (!fetchUrl(url, stream)) {
+    return false;
+  }
+  outContent = stream.c_str();
   return true;
 }
 

src/network/HttpDownloader.h

@@ -27,6 +27,8 @@ class HttpDownloader {
    */
   static bool fetchUrl(const std::string& url, std::string& outContent);
 
+  static bool fetchUrl(const std::string& url, Stream& stream);
+
   /**
    * Download a file to the SD card.
    * @param url The URL to download