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

Add a get_doc_handle virtual method to EvDocumentClass so the view layer
can obtain the underlying GepubDoc without depending on backend types
directly. This prepares for replacing EvWebView with GepubWidget which
needs the GepubDoc to render content.

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

WebKit2GTK renders MathML natively so the MathJax script injection is no
longer needed. Remove the mathjax-directory build option, the
auto-detection logic, and the CI package dependency.

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

Remove the bundled minizip directory, unused includes, and the night
mode virtual methods from EvDocumentClass which are no longer called by
any code. Night mode is now handled entirely through WebKit user
stylesheets.

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

Pre-compute all page URI paths at load time so get_page is thread-safe
and does not mutate GepubDoc state. Remove the EvDocumentFind interface
because the threaded find job is incompatible with the single-threaded
GepubDoc and WebKit. EPUB search will be reimplemented using WebKit's
native find controller in a future commit.

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

Add libgepub as a required dependency for the EPUB backend in both meson
and autotools build systems. No source code changes yet, but this
prepares for replacing the hand-rolled EPUB parsing with libgepub.

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

EPUB files with missing or incomplete metadata (like no title or missing
navigation elements) can cause NULL pointer dereferences in the XML
parsing functions. This adds NULL checks to xml_parse_children_of_node,
get_child_list, and the TOC text parsing loop.

Fixes #707