modules: split services and the ircd link into external crates

nickserv, chanserv and the InspIRCd protocol move out of the binary into their
own workspace crates (fedserv-nickserv, fedserv-chanserv, fedserv-inspircd),
each depending only on fedserv-api. human_time and the branded account emails
move into the SDK crate so a module needs nothing from core; the engine keeps
its own inherent methods and builds emails via fedserv-api too. The bin now
constructs each module from its crate instead of an in-tree #[path] include.
Proves the SDK is self-sufficient: a third-party module is the same shape.

api/src/email.rs

@@ -0,0 +1,76 @@
+// Email content: renders the HTML templates in ../templates/email and pairs each
+// with a plaintext fallback. The link layer sends both as multipart/alternative.
+
+pub struct Mail {
+    pub subject: String,
+    pub text: String,
+    pub html: String,
+}
+
+const BASE: &str = include_str!("../templates/email/base.html");
+
+// The masthead: a logo image beside the brand name when a logo URL is set,
+// otherwise the brand name as an accent eyebrow.
+fn brand_mark(brand: &str, accent: &str, logo: &str) -> String {
+    let (brand, accent) = (escape(brand), escape(accent));
+    if logo.is_empty() {
+        format!("<span style=\"font-size:12px;font-weight:700;letter-spacing:1.6px;text-transform:uppercase;color:{accent};\">{brand}</span>")
+    } else {
+        format!(
+            "<img src=\"{}\" width=\"40\" height=\"40\" alt=\"\" style=\"display:inline-block;border-radius:9px;vertical-align:middle;\"><span style=\"display:inline-block;margin-left:12px;vertical-align:middle;font-size:18px;font-weight:800;letter-spacing:.2px;color:#0f172a;\">{brand}</span>",
+            escape(logo)
+        )
+    }
+}
+
+fn render(brand: &str, accent: &str, logo: &str, title: &str, message: &str, code: &str, note: &str) -> String {
+    BASE.replace("{{brand_mark}}", &brand_mark(brand, accent, logo))
+        .replace("{{brand}}", &escape(brand))
+        .replace("{{accent}}", &escape(accent))
+        .replace("{{title}}", &escape(title))
+        .replace("{{message}}", &escape(message))
+        .replace("{{code}}", &escape(code))
+        .replace("{{note}}", &escape(note))
+}
+
+pub fn reset(brand: &str, accent: &str, logo: &str, account: &str, code: &str) -> Mail {
+    Mail {
+        subject: format!("Password reset for {account}"),
+        text: format!(
+            "Your password reset code for {account} is: {code}\nIt expires in 15 minutes.\nReset with:\n  /msg NickServ RESETPASS {account} {code} <newpassword>\n"
+        ),
+        html: render(
+            brand,
+            accent,
+            logo,
+            "Password reset",
+            &format!("Use this code to reset the password for your account {account}."),
+            code,
+            "This code expires in 15 minutes. If you didn't ask to reset it, ignore this email.",
+        ),
+    }
+}
+
+pub fn confirm(brand: &str, accent: &str, logo: &str, account: &str, code: &str) -> Mail {
+    Mail {
+        subject: format!("Confirm your {account} registration"),
+        text: format!(
+            "Confirm your account {account} with:\n  /msg NickServ CONFIRM {code}\nThe code expires in 15 minutes.\n"
+        ),
+        html: render(
+            brand,
+            accent,
+            logo,
+            "Confirm your account",
+            &format!("Welcome! Confirm the email for your account {account} with the code below."),
+            code,
+            "This code expires in 15 minutes.",
+        ),
+    }
+}
+
+// Escape the few characters that matter inside HTML text so a value can't break
+// out of the template.
+fn escape(s: &str) -> String {
+    s.replace('&', "&amp;").replace('<', "&lt;").replace('>', "&gt;")
+}