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

lib/email-composer-utils.ts

@@ -0,0 +1,23 @@
+const HTML_ESCAPE_MAP = {
+  "&": "&",
+  "<": "&lt;",
+  ">": "&gt;",
+  '"': "&quot;",
+  "'": "&#39;",
+} as const;
+
+function escapeHtml(value: string): string {
+  return value.replace(/[&<>"']/g, (char) =>
+    HTML_ESCAPE_MAP[char as keyof typeof HTML_ESCAPE_MAP]
+  );
+}
+
+export function plainTextToComposerBody(text: string): string {
+  if (!text) return "";
+
+  return text
+    .replace(/\r\n?/g, "\n")
+    .split(/\n{2,}/)
+    .map((paragraph) => `<p>${escapeHtml(paragraph).replace(/\n/g, "<br>")}</p>`)
+    .join("");
+}