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/HttpDownloader.h

@@ -20,22 +20,18 @@ class HttpDownloader {
   };
 
   /**
-   * Fetch text content from a URL.
-   * @param url The URL to fetch
-   * @param outContent The fetched content (output)
-   * @return true if fetch succeeded, false on error
+   * Fetch text content from a URL with optional credentials.
    */
-  static bool fetchUrl(const std::string& url, std::string& outContent);
+  static bool fetchUrl(const std::string& url, std::string& outContent, const std::string& username = "",
+                       const std::string& password = "");
 
-  static bool fetchUrl(const std::string& url, Stream& stream);
+  static bool fetchUrl(const std::string& url, Stream& stream, const std::string& username = "",
+                       const std::string& password = "");
 
   /**
-   * Download a file to the SD card.
-   * @param url The URL to download
-   * @param destPath The destination path on SD card
-   * @param progress Optional progress callback
-   * @return DownloadError indicating success or failure type
+   * Download a file to the SD card with optional credentials.
    */
   static DownloadError downloadToFile(const std::string& url, const std::string& destPath,
-                                      ProgressCallback progress = nullptr);
+                                      ProgressCallback progress = nullptr, const std::string& username = "",
+                                      const std::string& password = "");
 };