Feature/protocol handlers

* Added account selection for protocol links when multiple connected accounts are available, including mailto: links
* Added support for handling mailto: links in an already-open PWA/session instead of always opening a new tab
* Added webcal: protocol handling for calendar links
* Added account selection for webcal: links when multiple calendar-capable accounts are connected
* Added an import-or-subscribe choice for detected webcal calendars
* Added protocol handler settings for registering mail and calendar handlers and choosing the open mode
* Added service worker/session coordination for passing protocol requests between browser/PWA contexts
* Added tests and translations for the new protocol handler flows

app/protocol/mailto/page.tsx

@@ -0,0 +1,8 @@
+import { getTranslations } from "next-intl/server";
+import { MailtoProtocolClient } from "@/components/protocol/mailto-protocol-client";
+
+export default async function MailtoProtocolPage() {
+  const t = await getTranslations("protocol_handlers");
+
+  return <MailtoProtocolClient openingText={t("opening_mailto")} />;
+}

app/protocol/webcal/page.tsx

@@ -0,0 +1,8 @@
+import { getTranslations } from "next-intl/server";
+import { WebcalProtocolClient } from "@/components/protocol/webcal-protocol-client";
+
+export default async function WebcalProtocolPage() {
+  const t = await getTranslations("protocol_handlers");
+
+  return <WebcalProtocolClient openingText={t("opening_webcal")} />;
+}