Offer ihasmail as the browser's mailto: handler

Settings > General gains a "Default mail app" section that calls
registerProtocolHandler so mail links anywhere in the browser open ihasmail.
The browser owns the decision and there is no API to read it back, so the UI
says what it can: it records that we asked, offers "Ask again", shows a
Remove button where unregisterProtocolHandler exists, and points at the
browser's own settings. Unsupported browsers (Safari) and insecure contexts
get an explanation instead of a dead button.

The manifest now declares protocol_handlers for mailto, which is the route by
which an *installed* app can be offered by the operating system itself; the
UI says so and links the two ideas rather than promising a system-wide
default the page cannot grant.

Mailto parsing is now one function (parseMailto in lib/address.ts) instead of
three hand-rolled copies in AppShell and MessageView. It follows RFC 6068:
recipients from the path, the to= header or both, case-insensitive headers,
"+" as space, and tolerant of malformed escapes. That fixes Cc and Bcc being
silently dropped, and draftFromMailto escapes the body so a mailto: URL from
an untrusted page reaches the composer as text rather than markup.

web/src/views/settings/GeneralSettings.tsx

@@ -2,6 +2,15 @@ import { useSettings } from "@/store/settings";
 import { Switch } from "@/ui/misc";
 import { browserTimeZone, listTimeZones } from "@/lib/dates";
 import { toast } from "@/ui/toast";
+import { useState } from "react";
+import {
+  canUnregisterMailtoHandler,
+  isInstalledApp,
+  mailtoHandlerRequested,
+  mailtoHandlerSupport,
+  registerMailtoHandler,
+  unregisterMailtoHandler,
+} from "@/lib/mailhandler";
 import {
   browserLocale,
   formatClock,
@@ -154,6 +163,9 @@ export function GeneralSettings() {
       </div>
       <p className="hint">Preview: {formatFullDateTime(SAMPLE)}</p>
 
+      <h2>Default mail app</h2>
+      <MailHandlerSettings />
+
       <h2>Backup</h2>
       <div className="row wrap">
         <button className="btn" onClick={() => { const blob = new Blob([exportJson()], { type: "application/json" }); const a = document.createElement("a"); a.href = URL.createObjectURL(blob); a.download = "ihasmail-settings.json"; a.click(); }}>Export settings</button>
@@ -167,3 +179,54 @@ export function GeneralSettings() {
   );
 }
 
+/**
+ * Offer ihasmail as the browser's handler for `mailto:` links. The browser
+ * owns the decision, and nothing can read the answer back, so this states what
+ * it can and points at the browser's own settings for the rest.
+ */
+function MailHandlerSettings() {
+  const support = mailtoHandlerSupport();
+  const [requested, setRequested] = useState(mailtoHandlerRequested);
+
+  const ask = () => {
+    try {
+      registerMailtoHandler();
+      setRequested(true);
+      toast.success("Your browser will ask whether to open mail links in ihasmail");
+    } catch (err) {
+      toast.error(`Your browser refused the request: ${(err as Error).message}`);
+    }
+  };
+
+  const remove = () => {
+    unregisterMailtoHandler();
+    setRequested(false);
+    toast.show("Removed. Mail links will open in whatever your browser falls back to.");
+  };
+
+  if (support === "unsupported") {
+    return <p className="hint">This browser cannot register apps for <code>mailto:</code> links. Safari, in particular, has no such API — you can still make ihasmail the default from your operating system if you install it as an app.</p>;
+  }
+  if (support === "insecure") {
+    return <p className="hint">Registering for <code>mailto:</code> links requires a secure (HTTPS) connection.</p>;
+  }
+
+  return (
+    <>
+      <p className="hint">
+        Open <code>mailto:</code> links — in web pages, documents and other apps — in ihasmail instead of a desktop mail client.
+        Your browser will ask you to confirm, and you can change it later in its own settings (Chrome: Settings › Privacy and security › Site settings › Protocol handlers; Firefox: Settings › General › Applications).
+      </p>
+      <div className="row wrap">
+        <button className="btn btn-primary" onClick={ask}>{requested ? "Ask again" : "Make ihasmail the default mail app"}</button>
+        {requested && canUnregisterMailtoHandler() && <button className="btn btn-ghost" onClick={remove}>Remove</button>}
+      </div>
+      {requested && <p className="hint mt-8">Requested in this browser. Whether it took effect is up to the browser — check its settings if mail links still open elsewhere.</p>}
+      {!isInstalledApp() && (
+        <p className="hint mt-8">
+          For a system-wide default, install ihasmail as an app first (in Chrome: the install icon in the address bar). Your operating system can then offer ihasmail directly wherever it asks which mail app to use.
+        </p>
+      )}
+    </>
+  );
+}