Calibre Web Epub Downloading + Calibre Wireless Device Syncing (#219)

## Summary

Adds support for browsing and downloading books from a Calibre-web
server via OPDS.
How it works
1. Configure server URL in Settings → Calibre Web URL (e.g.,
https://myserver.com:port I use Cloudflare tunnel to make my server
accessible anywhere fwiw)
2. "Calibre Library" will now show on the the home screen
3. Browse the catalog - navigate through categories like "By Newest",
"By Author", "By Series", etc.
4. Download books - select a book and press Confirm to download the EPUB
to your device
Navigation
- Up/Down - Move through entries
- Confirm - Open folder or download book
- Back - Go to parent catalog, or exit to home if at root
- Navigation entries show with > prefix, books show title and author
- Button hints update dynamically ("Open" for folders, "Download" for
books)
Technical details
- Fetches OPDS catalog from {server_url}/opds
- Parses both navigation feeds (catalog links) and acquisition feeds
(downloadable books)
- Maintains navigation history stack for back navigation
- Handles absolute paths in OPDS links correctly (e.g.,
/books/opds/navcatalog/...)
- Downloads EPUBs directly to the SD card root
Note
The server URL should be typed to include https:// if the server
requires it - HTTP→HTTPS redirects may cause SSL errors on ESP32.

## Additional Context

* I also changed the home titles to use uppercase for each word and
added a setting to change the size of the side margins

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

lib/OpdsParser/OpdsParser.h

@@ -0,0 +1,99 @@
+#pragma once
+#include <expat.h>
+
+#include <string>
+#include <vector>
+
+/**
+ * Type of OPDS entry.
+ */
+enum class OpdsEntryType {
+  NAVIGATION,  // Link to another catalog
+  BOOK         // Downloadable book
+};
+
+/**
+ * Represents an entry from an OPDS feed (either a navigation link or a book).
+ */
+struct OpdsEntry {
+  OpdsEntryType type = OpdsEntryType::NAVIGATION;
+  std::string title;
+  std::string author;  // Only for books
+  std::string href;    // Navigation URL or epub download URL
+  std::string id;
+};
+
+// Legacy alias for backward compatibility
+using OpdsBook = OpdsEntry;
+
+/**
+ * Parser for OPDS (Open Publication Distribution System) Atom feeds.
+ * Uses the Expat XML parser to parse OPDS catalog entries.
+ *
+ * Usage:
+ *   OpdsParser parser;
+ *   if (parser.parse(xmlData, xmlLength)) {
+ *     for (const auto& entry : parser.getEntries()) {
+ *       if (entry.type == OpdsEntryType::BOOK) {
+ *         // Downloadable book
+ *       } else {
+ *         // Navigation link to another catalog
+ *       }
+ *     }
+ *   }
+ */
+class OpdsParser {
+ public:
+  OpdsParser() = default;
+  ~OpdsParser();
+
+  // Disable copy
+  OpdsParser(const OpdsParser&) = delete;
+  OpdsParser& operator=(const OpdsParser&) = delete;
+
+  /**
+   * Parse an OPDS XML feed.
+   * @param xmlData Pointer to the XML data
+   * @param length Length of the XML data
+   * @return true if parsing succeeded, false on error
+   */
+  bool parse(const char* xmlData, size_t length);
+
+  /**
+   * Get the parsed entries (both navigation and book entries).
+   * @return Vector of OpdsEntry entries
+   */
+  const std::vector<OpdsEntry>& getEntries() const { return entries; }
+
+  /**
+   * Get only book entries (legacy compatibility).
+   * @return Vector of book entries
+   */
+  std::vector<OpdsEntry> getBooks() const;
+
+  /**
+   * Clear all parsed entries.
+   */
+  void clear();
+
+ private:
+  // Expat callbacks
+  static void XMLCALL startElement(void* userData, const XML_Char* name, const XML_Char** atts);
+  static void XMLCALL endElement(void* userData, const XML_Char* name);
+  static void XMLCALL characterData(void* userData, const XML_Char* s, int len);
+
+  // Helper to find attribute value
+  static const char* findAttribute(const XML_Char** atts, const char* name);
+
+  XML_Parser parser = nullptr;
+  std::vector<OpdsEntry> entries;
+  OpdsEntry currentEntry;
+  std::string currentText;
+
+  // Parser state
+  bool inEntry = false;
+  bool inTitle = false;
+  bool inAuthor = false;
+  bool inAuthorName = false;
+  bool inId = false;
+};