feat(email): replace unicode checkmark with inline PNG image (#172)

The signer-verified circle previously rendered a unicode `✓`
glyph which is styled inconsistently across mail clients. Embed a
PNG of the PostGuard checkmark via `cid:pg-check` to match the
shape used in the download page UI.

Author: rubenhensen

src/email.rs

@@ -109,6 +109,11 @@ fn take_firstname_lastname_pair(attrs: &mut Vec<(String, String)>) -> Option<Str
 /// HTML-only-plus-remote-image spam signal flagged in postguard#197.
 const LOGO_PNG: &[u8] = include_bytes!("../templates/email/pg_logo.png");
 
+/// Inline checkmark glyph used inside the signer-verified circle in the
+/// HTML email, referenced via `cid:pg-check`. Replaces the previous
+/// unicode `&#10003;` so the mark renders consistently across clients.
+const CHECK_PNG: &[u8] = include_bytes!("../templates/email/check.png");
+
 use serde::{Deserialize, Serialize};
 use url::Url;
 
@@ -207,10 +212,13 @@ struct EmailTextTemplate<'a> {
 fn build_body(html: String, text: String) -> Result<MultiPart, Box<dyn std::error::Error>> {
     let logo = Attachment::new_inline("pg-logo".to_string())
         .body(LOGO_PNG.to_vec(), "image/png".parse::<ContentType>()?);
+    let check = Attachment::new_inline("pg-check".to_string())
+        .body(CHECK_PNG.to_vec(), "image/png".parse::<ContentType>()?);
 
     let related = MultiPart::related()
         .singlepart(SinglePart::html(html))
-        .singlepart(logo);
+        .singlepart(logo)
+        .singlepart(check);
 
     Ok(MultiPart::alternative()
         .singlepart(SinglePart::plain(text))

templates/email/check.png

@@ -42,7 +42,9 @@
                 {% if sender_email != "" %}
                 <div style="margin-top:40px;padding-top:30px;border-top:1px solid #C6E2F6;text-align:center;">
                     <div style="margin-bottom:12px;">
-                        <span style="display:inline-block;width:32px;height:32px;line-height:32px;border-radius:50%;border:2px solid #5F7381;text-align:center;font-size:16px;color:#5F7381;box-sizing:border-box;">&#10003;</span>
+                        <span style="display:inline-block;width:32px;height:32px;line-height:32px;border-radius:50%;border:2px solid #5F7381;text-align:center;box-sizing:border-box;">
+                            <img src="cid:pg-check" alt="" width="16" height="13" style="display:inline-block;vertical-align:middle;" />
+                        </span>
                     </div>
                     <p style="font-size:13px;color:#5F7381;margin:0 0 6px 0;">{{files_from}}</p>
                     <p style="font-size:15px;font-weight:700;color:#030E17;margin:0 0 12px 0;">{{sender_email}}</p>