feat: support two-step OpenSearch Description for OPDS search

Servers like copyparty advertise search via a link to an OpenSearch
Description (OSD) document rather than embedding {searchTerms} directly
in the feed's <link rel="search"> href (the Calibre-Web style).

OpdsParser now stores the OSD URL when rel="search" and
type="application/opensearchdescription+xml" but the href lacks
{searchTerms}. OpdsBookBrowserActivity fetches that OSD document after
each feed load and parses the <Url template="..."> element with a
minimal inline expat parser to extract the actual search template.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/OpdsParser/OpdsParser.cpp

@@ -70,6 +70,7 @@ bool OpdsParser::error() const { return errorOccured; }
 void OpdsParser::clear() {
   entries.clear();
   searchTemplate.clear();
+  osdUrl.clear();
   nextPageUrl.clear();
   prevPageUrl.clear();
   currentEntry = OpdsEntry{};
@@ -105,6 +106,8 @@ void XMLCALL OpdsParser::startElement(void* userData, const XML_Char* name, cons
         std::string sHref(href);
         if (sHref.find("{searchTerms}") != std::string::npos) {
           self->searchTemplate = sHref;
+        } else if (type && strcmp(type, "application/opensearchdescription+xml") == 0) {
+          self->osdUrl = sHref;
         }
       } else if (rel && strcmp(rel, "next") == 0 && !self->inEntry) {
         self->nextPageUrl = href;

lib/OpdsParser/OpdsParser.h

@@ -50,6 +50,7 @@ class OpdsParser final : public Print {
 
   // Disable copy
   const std::string& getSearchTemplate() const { return searchTemplate; }
+  const std::string& getOsdUrl() const { return osdUrl; }
   const std::string& getNextPageUrl() const { return nextPageUrl; }
   const std::string& getPrevPageUrl() const { return prevPageUrl; }
   OpdsParser(const OpdsParser&) = delete;
@@ -89,6 +90,7 @@ class OpdsParser final : public Print {
   static void XMLCALL characterData(void* userData, const XML_Char* s, int len);
 
   std::string searchTemplate;
+  std::string osdUrl;
   std::string nextPageUrl;
   std::string prevPageUrl;
   // Helper to find attribute value

src/activities/browser/OpdsBookBrowserActivity.cpp

@@ -7,6 +7,7 @@
 #include <Logging.h>
 #include <OpdsStream.h>
 #include <WiFi.h>
+#include <expat.h>
 
 #include <cctype>
 #include <cstdio>
@@ -245,6 +246,9 @@ void OpdsBookBrowserActivity::fetchFeed(const std::string& path) {
   }
 
   searchTemplate = parser.getSearchTemplate();
+  if (searchTemplate.empty() && !parser.getOsdUrl().empty()) {
+    fetchOsdTemplate(UrlUtils::buildUrl(url, parser.getOsdUrl()));
+  }
   const auto& nextUrl = parser.getNextPageUrl();
   const auto& prevUrl = parser.getPrevPageUrl();
   entries = std::move(parser).getEntries();
@@ -336,6 +340,42 @@ void OpdsBookBrowserActivity::downloadBook(const OpdsEntry& book) {
   }
 }
 
+void OpdsBookBrowserActivity::fetchOsdTemplate(const std::string& osdUrl) {
+  std::string content;
+  if (!HttpDownloader::fetchUrl(osdUrl, content)) {
+    LOG_ERR("OPDS", "Failed to fetch OSD: %s", osdUrl.c_str());
+    return;
+  }
+
+  struct OsdState {
+    std::string templateUrl;
+    static void XMLCALL onStart(void* ud, const XML_Char* name, const XML_Char** atts) {
+      if (strcmp(name, "Url") != 0 && strstr(name, ":Url") == nullptr) return;
+      auto* state = static_cast<OsdState*>(ud);
+      for (int i = 0; atts[i]; i += 2) {
+        if (strcmp(atts[i], "template") == 0 && strstr(atts[i + 1], "{searchTerms}") != nullptr) {
+          state->templateUrl = atts[i + 1];
+          break;
+        }
+      }
+    }
+  } osdState;
+
+  XML_Parser p = XML_ParserCreate(nullptr);
+  if (!p) {
+    LOG_ERR("OPDS", "OSD parser alloc failed");
+    return;
+  }
+  XML_SetUserData(p, &osdState);
+  XML_SetElementHandler(p, OsdState::onStart, nullptr);
+  XML_Parse(p, content.c_str(), static_cast<int>(content.size()), XML_TRUE);
+  XML_ParserFree(p);
+
+  if (!osdState.templateUrl.empty()) {
+    searchTemplate = UrlUtils::buildUrl(osdUrl, osdState.templateUrl);
+  }
+}
+
 void OpdsBookBrowserActivity::launchSearch() {
   consumeConfirm = true;
   state = BrowserState::SEARCH_INPUT;

src/activities/browser/OpdsBookBrowserActivity.h

@@ -46,6 +46,7 @@ class OpdsBookBrowserActivity final : public Activity {
   void navigateToEntry(const OpdsEntry& entry);
   void navigateBack();
   void downloadBook(const OpdsEntry& book);
+  void fetchOsdTemplate(const std::string& osdUrl);
   void launchSearch();
   void performSearch(const std::string& query);
   bool preventAutoSleep() override { return true; }