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/protocol-handlers/webcal.ts

@@ -0,0 +1,48 @@
+export interface ParsedWebcal {
+  originalUrl: string;
+  subscriptionUrl: string;
+  suggestedName: string;
+}
+
+function stripControlChars(value: string): string {
+  return value.replace(/[\u0000-\u001F\u007F]/g, "").trim();
+}
+
+function extensionlessName(value: string): string {
+  return value.replace(/\.(ics|ical)$/i, "");
+}
+
+function decodePathSegment(value: string): string {
+  try {
+    return decodeURIComponent(value);
+  } catch {
+    return value;
+  }
+}
+
+export function parseWebcal(raw: string): ParsedWebcal | null {
+  let url: URL;
+
+  try {
+    url = new URL(raw);
+  } catch {
+    return null;
+  }
+
+  if (url.protocol === "webcal:" || url.protocol === "webcals:") {
+    url = new URL(raw.replace(/^webcals?:/i, "https:"));
+  } else if (url.protocol !== "http:" && url.protocol !== "https:") {
+    return null;
+  }
+
+  const subscriptionUrl = url.toString();
+  const queryName = stripControlChars(url.searchParams.get("name") || "");
+  const pathSegment = stripControlChars(decodePathSegment(url.pathname.split("/").filter(Boolean).pop() || ""));
+  const suggestedName = queryName || extensionlessName(pathSegment) || url.hostname;
+
+  return {
+    originalUrl: raw,
+    subscriptionUrl,
+    suggestedName,
+  };
+}