Security: refuse mailto attachments, show link hosts in punycode (follow-up to #104) (#337)

* Security: refuse email links that attach or insert a local file

Follow-up to #104. A document's mailto: link could ask the email app to
attach a file from the user's computer (Evolution, KMail and Claws Mail
honour attach=, Evolution and KMail also attachment=) or read one into
the body (Claws Mail's insert=).

document_url now refuses a mailto: address in which any word starting
with attach or insert (any case) is followed by =, wherever it stands
and after each layer of percent-decoding. It also refuses text that
some email tools decode before reading the address: RFC 2047 encoded
words, which KMail decodes across the whole address, and backslash
escapes, which xdg-email's echo decodes. The link is refused rather
than rewritten: PdfCraft doesn't change what a document asks to open,
and the email apps' parsers disagree with one another.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Security: show a link's host as the browser receives it, in punycode

Follow-up to #104. The link prompt's host now comes from
links::display_host, which reads it with url::Url: the webbrowser crate
hands the OS that crate's serialisation, and the web build's browser
parses by the same WHATWG standard. International hosts are shown in
punycode with a note, a label mixing writing systems (UTS #39 resolved
script sets) gets a red warning, numeric hosts such as 3232235777 show
as 192.168.1.1, and a host the browser would reject is refused as
malformed. The Unicode form isn't repeated in the prompt, so a
whole-script lookalike can't read as the real site.

url, idna and icu_properties become direct dependencies of
pdfcraft-engine; all three were already in the graph.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Security: refuse document addresses the URL parser rejects

Follow-up to #104. egui opens links through the webbrowser crate, which
parses the address with url::Url and, when that fails, treats it as a
local file path and opens file:///<cwd>/<address> in the browser.
document_url accepted addresses that don't parse, such as
mailto://a^b/x or https://a^b/ (a forbidden host code point), so a
document could still get the browser to open a local path.

document_url now also requires url::Url to accept the address with the
same scheme, which is exactly the parse webbrowser does first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

parity/acrobat-features.toml

@@ -2202,8 +2202,8 @@ milestone = "M7"
 status = "shipped"
 commands = ["edit.link"]
 tools = ["link_add"]
-evidence = ["crates/annot/src/tests.rs::links_are_added_edited_listed_and_removed", "crates/automation/tests/automation.rs::links_through_tools", "crates/ui-egui/tests/document_links.rs::a_link_asks_first_and_opens_when_allowed", "crates/ui-egui/tests/document_links.rs::a_button_uri_action_asks_first"]
-notes = "Opening a document's link or button URI asks first; only http, https and mailto addresses are offered (#90, #91)."
+evidence = ["crates/annot/src/tests.rs::links_are_added_edited_listed_and_removed", "crates/automation/tests/automation.rs::links_through_tools", "crates/ui-egui/tests/document_links.rs::a_link_asks_first_and_opens_when_allowed", "crates/ui-egui/tests/document_links.rs::a_button_uri_action_asks_first", "crates/ui-egui/tests/document_links.rs::an_email_link_cannot_attach_a_local_file", "crates/ui-egui/tests/document_links.rs::a_lookalike_host_is_shown_in_punycode_and_flagged"]
+notes = "Opening a document's link or button URI asks first; only http, https and mailto addresses are offered (#90, #91), and an email address that asks the email app to attach or insert a local file is refused. The prompt shows the host as the browser receives it: international hosts in punycode, lookalikes that mix alphabets flagged, numeric hosts in their usual form."
 
 [[feature]]
 id = "edit.action-launch"