feat: Support for multiple OPDS servers (#1209)

* 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"
/>

* 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.

---

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/HttpDownloader.cpp

@@ -11,7 +11,6 @@
 #include <memory>
 #include <utility>
 
-#include "CrossPointSettings.h"
 #include "util/UrlUtils.h"
 
 namespace {
@@ -52,8 +51,8 @@ class FileWriteStream final : public Stream {
 };
 }  // namespace
 
-bool HttpDownloader::fetchUrl(const std::string& url, Stream& outContent) {
-  // Use NetworkClientSecure for HTTPS, regular NetworkClient for HTTP
+bool HttpDownloader::fetchUrl(const std::string& url, Stream& outContent, const std::string& username,
+                              const std::string& password) {
   std::unique_ptr<NetworkClient> client;
   if (UrlUtils::isHttpsUrl(url)) {
     auto* secureClient = new NetworkClientSecure();
@@ -71,9 +70,8 @@ bool HttpDownloader::fetchUrl(const std::string& url, Stream& outContent) {
   http.setTimeout(30000);
   http.addHeader("User-Agent", "CrossPoint-ESP32-" CROSSPOINT_VERSION);
 
-  // Add Basic HTTP auth if credentials are configured
-  if (strlen(SETTINGS.opdsUsername) > 0 && strlen(SETTINGS.opdsPassword) > 0) {
-    std::string credentials = std::string(SETTINGS.opdsUsername) + ":" + SETTINGS.opdsPassword;
+  if (!username.empty() && !password.empty()) {
+    std::string credentials = username + ":" + password;
     String encoded = base64::encode(credentials.c_str());
     http.addHeader("Authorization", "Basic " + encoded);
   }
@@ -93,9 +91,10 @@ bool HttpDownloader::fetchUrl(const std::string& url, Stream& outContent) {
   return true;
 }
 
-bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
+bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent, const std::string& username,
+                              const std::string& password) {
   StreamString stream;
-  if (!fetchUrl(url, stream)) {
+  if (!fetchUrl(url, stream, username, password)) {
     return false;
   }
   outContent = stream.c_str();
@@ -103,8 +102,8 @@ bool HttpDownloader::fetchUrl(const std::string& url, std::string& outContent) {
 }
 
 HttpDownloader::DownloadError HttpDownloader::downloadToFile(const std::string& url, const std::string& destPath,
-                                                             ProgressCallback progress) {
-  // Use NetworkClientSecure for HTTPS, regular NetworkClient for HTTP
+                                                             ProgressCallback progress, const std::string& username,
+                                                             const std::string& password) {
   std::unique_ptr<NetworkClient> client;
   if (UrlUtils::isHttpsUrl(url)) {
     auto* secureClient = new NetworkClientSecure();
@@ -123,9 +122,8 @@ HttpDownloader::DownloadError HttpDownloader::downloadToFile(const std::string&
   http.setTimeout(65535);  // max uint16_t (~65s) — HTTPClient::setTimeout takes uint16_t ms
   http.addHeader("User-Agent", "CrossPoint-ESP32-" CROSSPOINT_VERSION);
 
-  // Add Basic HTTP auth if credentials are configured
-  if (strlen(SETTINGS.opdsUsername) > 0 && strlen(SETTINGS.opdsPassword) > 0) {
-    std::string credentials = std::string(SETTINGS.opdsUsername) + ":" + SETTINGS.opdsPassword;
+  if (!username.empty() && !password.empty()) {
+    std::string credentials = username + ":" + password;
     String encoded = base64::encode(credentials.c_str());
     http.addHeader("Authorization", "Basic " + encoded);
   }