feat: Support for multiple OPDS servers (#1209)

## Summary

* Add support for configuring and using multiple OPDS servers, replacing
the previous single-server limitation. Closes
https://github.com/crosspoint-reader/crosspoint-reader/issues/1178
* New OpdsServerStore singleton (modeled after WifiCredentialStore) that
persists up to 8 OPDS servers to /.crosspoint/opds.json with MAC-based
password obfuscation.
* One-time migration from legacy single-server fields in
CrossPointSettings to the new store on first boot.
* New OpdsServerListActivity for the device UI — works in two modes: a
settings list (add/edit/delete servers) and a picker (select which
server to browse). When only one server is configured, the picker is
skipped automatically.
* Renamed CalibreSettingsActivity → OpdsSettingsActivity for clarity. It
now edits individual OpdsServer entries (name, URL, username, password,
delete).
* OpdsBookBrowserActivity now receives an OpdsServer at construction and
uses its credentials for all fetches/downloads, and shows the server
name in the header.
* HttpDownloader::fetchUrl and downloadToFile accept optional per-call
username/password parameters instead of reading from global settings.
* REST API endpoints on CrossPointWebServer: GET /api/opds, POST
/api/opds, POST /api/opds/delete — passwords are never exposed over the
API (only a hasPassword flag), and omitting the password field on update
preserves the existing one.
* Web UI (SettingsPage.html) with dynamic OPDS server management cards —
add, edit, save, and delete servers from the browser.
<img width="932" height="906" alt="SCR-20260416-stvu"
src="https://github.com/user-attachments/assets/a8f18d84-4204-46a0-bb31-b73d24b3255f"
/>


## Additional Context

* The OpdsServerStore JSON format and obfuscation scheme are identical
to WifiCredentialStore, so the same JsonSettingsIO infrastructure
handles both.
* The web API uses POST /api/opds/delete instead of DELETE /api/opds
because the ESP32 WebServer doesn't support the DELETE method with a
request body.
* Existing single-server configurations are migrated automatically — no
user action required. After migration the legacy CrossPointSettings
fields are cleared so it only runs once.
* The HttpDownloader changes are backward-compatible: the credential
parameters default to empty strings, so existing callers are unaffected.

---

### 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? _**< YES >**_

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/network/CrossPointWebServer.cpp

@@ -11,6 +11,7 @@
 #include <algorithm>
 
 #include "CrossPointSettings.h"
+#include "OpdsServerStore.h"
 #include "SettingsList.h"
 #include "WebDAVHandler.h"
 #include "html/FilesPageHtml.generated.h"
@@ -160,6 +161,11 @@ void CrossPointWebServer::begin() {
   server->on("/api/settings", HTTP_GET, [this] { handleGetSettings(); });
   server->on("/api/settings", HTTP_POST, [this] { handlePostSettings(); });
 
+  // OPDS server endpoints
+  server->on("/api/opds", HTTP_GET, [this] { handleGetOpdsServers(); });
+  server->on("/api/opds", HTTP_POST, [this] { handlePostOpdsServer(); });
+  server->on("/api/opds/delete", HTTP_POST, [this] { handleDeleteOpdsServer(); });
+
   server->onNotFound([this] { handleNotFound(); });
   LOG_DBG("WEB", "[MEM] Free heap after route setup: %d bytes", ESP.getFreeHeap());
 
@@ -1244,6 +1250,122 @@ void CrossPointWebServer::handlePostSettings() {
   server->send(200, "text/plain", String("Applied ") + String(applied) + " setting(s)");
 }
 
+// ---- OPDS Server API ----
+
+void CrossPointWebServer::handleGetOpdsServers() const {
+  const auto& servers = OPDS_STORE.getServers();
+
+  // Stream JSON array incrementally to avoid allocating the full response in memory
+  server->setContentLength(CONTENT_LENGTH_UNKNOWN);
+  server->send(200, "application/json", "");
+  server->sendContent("[");
+
+  char output[512];
+  constexpr size_t outputSize = sizeof(output);
+  JsonDocument doc;
+
+  for (size_t i = 0; i < servers.size(); i++) {
+    doc.clear();
+    doc["index"] = i;
+    doc["name"] = servers[i].name;
+    doc["url"] = servers[i].url;
+    doc["username"] = servers[i].username;
+    // Never expose passwords over the API — only indicate whether one is set
+    doc["hasPassword"] = !servers[i].password.empty();
+
+    const size_t written = serializeJson(doc, output, outputSize);
+    if (written >= outputSize) continue;
+
+    if (i > 0) server->sendContent(",");
+    server->sendContent(output);
+  }
+
+  server->sendContent("]");
+  server->sendContent("");
+  LOG_DBG("WEB", "Served OPDS servers API (%zu servers)", servers.size());
+}
+
+void CrossPointWebServer::handlePostOpdsServer() {
+  if (!server->hasArg("plain")) {
+    server->send(400, "text/plain", "Missing JSON body");
+    return;
+  }
+
+  const String body = server->arg("plain");
+  JsonDocument doc;
+  const DeserializationError err = deserializeJson(doc, body);
+  if (err) {
+    server->send(400, "text/plain", String("Invalid JSON: ") + err.c_str());
+    return;
+  }
+
+  OpdsServer opdsServer;
+  opdsServer.name = doc["name"] | std::string("");
+  opdsServer.url = doc["url"] | std::string("");
+  opdsServer.username = doc["username"] | std::string("");
+
+  // The password field is optional in the JSON payload. When absent (vs. present but empty),
+  // we preserve the existing password — the web UI omits it when the user hasn't changed it.
+  bool hasPasswordField = doc["password"].is<const char*>() || doc["password"].is<std::string>();
+  std::string password = doc["password"] | std::string("");
+
+  if (doc["index"].is<int>()) {
+    int idx = doc["index"].as<int>();
+    if (idx < 0 || idx >= static_cast<int>(OPDS_STORE.getCount())) {
+      server->send(400, "text/plain", "Invalid server index");
+      return;
+    }
+    // Preserve existing password if not explicitly provided
+    if (!hasPasswordField) {
+      const auto* existing = OPDS_STORE.getServer(static_cast<size_t>(idx));
+      if (existing) password = existing->password;
+    }
+    opdsServer.password = password;
+    OPDS_STORE.updateServer(static_cast<size_t>(idx), opdsServer);
+    LOG_DBG("WEB", "Updated OPDS server at index %d", idx);
+  } else {
+    opdsServer.password = password;
+    if (!OPDS_STORE.addServer(opdsServer)) {
+      server->send(400, "text/plain", "Cannot add server (limit reached)");
+      return;
+    }
+    LOG_DBG("WEB", "Added new OPDS server: %s", opdsServer.name.c_str());
+  }
+
+  server->send(200, "text/plain", "OK");
+}
+
+// Uses POST (not HTTP DELETE) because ESP32 WebServer doesn't support DELETE with body.
+void CrossPointWebServer::handleDeleteOpdsServer() {
+  if (!server->hasArg("plain")) {
+    server->send(400, "text/plain", "Missing JSON body");
+    return;
+  }
+
+  const String body = server->arg("plain");
+  JsonDocument doc;
+  const DeserializationError err = deserializeJson(doc, body);
+  if (err) {
+    server->send(400, "text/plain", String("Invalid JSON: ") + err.c_str());
+    return;
+  }
+
+  if (!doc["index"].is<int>()) {
+    server->send(400, "text/plain", "Missing index");
+    return;
+  }
+
+  int idx = doc["index"].as<int>();
+  if (idx < 0 || idx >= static_cast<int>(OPDS_STORE.getCount())) {
+    server->send(400, "text/plain", "Invalid server index");
+    return;
+  }
+
+  OPDS_STORE.removeServer(static_cast<size_t>(idx));
+  LOG_DBG("WEB", "Deleted OPDS server at index %d", idx);
+  server->send(200, "text/plain", "OK");
+}
+
 // WebSocket callback trampoline
 void CrossPointWebServer::wsEventCallback(uint8_t num, WStype_t type, uint8_t* payload, size_t length) {
   if (wsInstance) {