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/mailto.ts

@@ -0,0 +1,115 @@
+export interface ParsedMailto {
+  to: string[];
+  cc: string[];
+  bcc: string[];
+  subject: string;
+  body: string;
+}
+
+const MAX_RECIPIENTS = 200;
+const MAX_SUBJECT_LENGTH = 998;
+const MAX_BODY_LENGTH = 64 * 1024;
+const CONTROL_CHARS = /[\u0000-\u001F\u007F]/g;
+const CONTROL_CHARS_EXCEPT_LINE_BREAKS = /[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F]/g;
+
+function stripControlChars(value: string): string {
+  return value.replace(CONTROL_CHARS, "");
+}
+
+function stripBodyControlChars(value: string): string {
+  return value
+    .replace(/\r\n?/g, "\n")
+    .replace(CONTROL_CHARS_EXCEPT_LINE_BREAKS, "");
+}
+
+function splitRecipients(value: string): string[] {
+  return stripControlChars(value)
+    .split(",")
+    .map((recipient) => recipient.trim())
+    .filter(Boolean);
+}
+
+type QueryParam = {
+  key: string;
+  value: string;
+};
+
+function getQueryValue(searchParams: QueryParam[], key: string): string {
+  const values: string[] = [];
+  const lowerKey = key.toLowerCase();
+
+  for (const { key: paramKey, value } of searchParams) {
+    if (paramKey.toLowerCase() === lowerKey) {
+      values.push(value);
+    }
+  }
+
+  return values.join(",");
+}
+
+function decodePathname(pathname: string): string | null {
+  try {
+    return decodeURIComponent(pathname || "");
+  } catch {
+    return null;
+  }
+}
+
+function decodeQueryPart(value: string): string | null {
+  try {
+    // RFC 6068 uses percent-encoding for mailto query fields; unlike form
+    // encoding, a literal '+' is part of the value and must not become space.
+    return decodeURIComponent(value);
+  } catch {
+    return null;
+  }
+}
+
+function parseQuery(query: string): QueryParam[] | null {
+  if (!query) return [];
+
+  const params: QueryParam[] = [];
+  for (const part of query.split("&")) {
+    if (!part) continue;
+    const separatorIndex = part.indexOf("=");
+    const rawKey = separatorIndex >= 0 ? part.slice(0, separatorIndex) : part;
+    const rawValue = separatorIndex >= 0 ? part.slice(separatorIndex + 1) : "";
+    const key = decodeQueryPart(rawKey);
+    const value = decodeQueryPart(rawValue);
+    if (key === null || value === null) return null;
+    params.push({ key, value });
+  }
+
+  return params;
+}
+
+export function parseMailto(raw: string): ParsedMailto | null {
+  if (!raw.toLowerCase().startsWith("mailto:")) return null;
+
+  const addressAndQuery = raw.slice("mailto:".length);
+  const queryIndex = addressAndQuery.indexOf("?");
+  const rawPathname = queryIndex >= 0 ? addressAndQuery.slice(0, queryIndex) : addressAndQuery;
+  const rawQuery = queryIndex >= 0 ? addressAndQuery.slice(queryIndex + 1) : "";
+
+  const decodedPathname = decodePathname(rawPathname);
+  if (decodedPathname === null) return null;
+  const searchParams = parseQuery(rawQuery);
+  if (searchParams === null) return null;
+
+  const to = [
+    ...splitRecipients(decodedPathname),
+    ...splitRecipients(getQueryValue(searchParams, "to")),
+  ].slice(0, MAX_RECIPIENTS);
+  const remainingAfterTo = Math.max(0, MAX_RECIPIENTS - to.length);
+  const cc = splitRecipients(getQueryValue(searchParams, "cc")).slice(0, remainingAfterTo);
+  const remainingAfterCc = Math.max(0, MAX_RECIPIENTS - to.length - cc.length);
+  const bcc = splitRecipients(getQueryValue(searchParams, "bcc")).slice(0, remainingAfterCc);
+
+  return {
+    to,
+    cc,
+    bcc,
+    subject: stripControlChars(getQueryValue(searchParams, "subject")).slice(0, MAX_SUBJECT_LENGTH),
+    body: stripBodyControlChars(getQueryValue(searchParams, "body")).slice(0, MAX_BODY_LENGTH),
+  };
+}