Bypass the threaded EvJobFind for EPUB documents and use WebKit's
WebKitFindController directly. The threaded find job was incompatible
with GepubDoc's single-threaded design and caused segfaults. The new
approach searches within the current page using WebKit's built-in text
search and highlighting, which is both simpler and thread-safe.

Assisted-by: OpenCode:claude-opus-4-6

Store a reference to the GepubDoc in EvWebView via the new
get_doc_handle virtual method. Simplify next_page and previous_page to
just update the document model, letting the page-changed callback handle
content loading. This removes duplicate URI loading and prepares for
reimplementing search with WebKit's find controller.

Assisted-by: OpenCode:claude-opus-4-6

Replace the old approach of modifying extracted HTML files on disk with
a WebKitUserStyleSheet injected via the WebKitUserContentManager. This
is cleaner and works with the new libgepub backend which no longer
extracts files to a temporary directory.

Assisted-by: OpenCode:claude-opus-4-6

Replace the hand-rolled EPUB parsing with libgepub. This removes the
bundled minizip library, custom XML utilities, and temporary directory
extraction in favor of GepubDoc which handles archive access, metadata,
spine navigation, and table of contents. An epub:// URI scheme handler
serves resources directly from the archive without extracting to disk.
Search and night mode are stubbed as no-ops pending follow-up commits.

Assisted-by: OpenCode:claude-opus-4-6

The web-view callback for inverted colors got called, and if it did not find a web-view open
it defaulted to FALSE for inverted colors.

origin commit:
https://github.com/linuxmint/xreader/commit/abb5060
https://github.com/linuxmint/xreader/issues/38

origin commit:
https://github.com/linuxmint/xreader/commit/43acb8f