Since poppler 24.12 the PopplerColor channels of PopplerSigningData are
scaled from 16 bit to [0, 1]. Older versions pass them unscaled to
AnnotColor, which expects [0, 1], so any value above 1 resulted in a
black signature field. Scale by 65535 on newer poppler and round each
channel to 0 or 1 on older versions.

- guard signing API behind poppler 23.07, validation behind 22.02
- only enable Digital Signing when the backend can sign
- show signing errors, remove temp file on failure
- reload the window when the open document was overwritten and keep
  the original file permissions
- escape certificate names in markup, use a localized date
- draw the signature rect once, reset pressed button on release,
  handle rotation/borders and clamp to the page
- fix test/Makefile.am EXTRA_DIST

Backported from https://gitlab.gnome.org/GNOME/evince/-/commit/602d7148

Fixes #725

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

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