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>

USER_GUIDE.md

@@ -20,7 +20,8 @@ Welcome to the **CrossPoint** firmware. This guide outlines the hardware control
       - [3.6.2 Reader](#362-reader)
       - [3.6.3 Controls](#363-controls)
       - [3.6.4 System](#364-system)
-      - [3.6.5 KOReader Sync Quick Setup](#365-koreader-sync-quick-setup)
+      - [3.6.5 OPDS Servers (Multiple Libraries)](#365-opds-servers-multiple-libraries)
+      - [3.6.6 KOReader Sync Quick Setup](#366-koreader-sync-quick-setup)
     - [3.7 Sleep Screen](#37-sleep-screen)
   - [4. Reading Mode](#4-reading-mode)
     - [Page Turning](#page-turning)
@@ -194,12 +195,35 @@ The Settings screen allows you to configure the device's behavior. There are a f
 
 - **WiFi Networks**: Connect to WiFi networks for file transfers and firmware updates.
 - **KOReader Sync**: Options for setting up KOReader for syncing book progress.
-- **OPDS Browser**: Configure OPDS server settings for browsing and downloading books. Set the server URL (for Calibre Content Server, add `/opds` to the end), and optionally configure username and password for servers requiring authentication. Note: Only HTTP Basic authentication is supported. If using Calibre Content Server with authentication enabled, you must set it to use Basic authentication instead of the default Digest authentication.
+- **OPDS Servers**: Manage one or more OPDS libraries for browsing and downloading books. See [OPDS Servers (Multiple Libraries)](#365-opds-servers-multiple-libraries) below.
 - **Clear Reading Cache**: Clear the internal SD card cache.
 - **Check for updates**: Check for Crosspoint firmware updates over WiFi.
 - **Language**: Set the system language (see **[Supported Languages](#supported-languages)** for more information).
 
-#### 3.6.5 KOReader Sync Quick Setup
+#### 3.6.5 OPDS Servers (Multiple Libraries)
+
+CrossPoint supports saving multiple OPDS servers and switching between them when browsing catalogs.
+
+1. Open **Settings -> System -> OPDS Servers**.
+2. Select **Add Server** to create a new entry, or select an existing server to edit it.
+3. Configure these fields:
+  - **Server Name**: Optional display name (for example, "Home Calibre" or "Public Catalog").
+  - **OPDS Server URL**: Full catalog root URL (for Calibre Content Server, usually ends with `/opds`).
+  - **Username / Password**: Optional credentials for authenticated servers.
+4. Use **Delete Server** inside a server entry to remove it.
+
+Behavior notes:
+
+- You can store up to 8 OPDS servers.
+- OPDS authentication supports HTTP Basic auth. If you use Calibre Content Server with authentication enabled, set it to Basic (not Digest).
+
+You can also manage OPDS servers from the web interface while in File Transfer mode:
+
+1. Connect to the device web UI.
+2. Open `http://<device-ip>/settings`.
+3. Use the **OPDS Servers** card to add, edit, or delete entries.
+
+#### 3.6.6 KOReader Sync Quick Setup
 
 CrossPoint can sync reading progress with KOReader-compatible sync servers.
 It also interoperates with KOReader apps/devices when they use the same server and credentials.