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/__tests__/email-composer-utils.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import { plainTextToComposerBody } from "../email-composer-utils";
+
+describe("plainTextToComposerBody", () => {
+  it("returns an empty string for empty input", () => {
+    expect(plainTextToComposerBody("")).toBe("");
+  });
+
+  it("escapes HTML before building composer paragraphs", () => {
+    expect(plainTextToComposerBody("<script>alert('x') & \"q\"</script>")).toBe(
+      "<p>&lt;script&gt;alert(&#39;x&#39;) &amp; &quot;q&quot;&lt;/script&gt;</p>"
+    );
+  });
+
+  it("normalizes line endings and preserves single line breaks", () => {
+    expect(plainTextToComposerBody("line1\r\nline2\rline3")).toBe(
+      "<p>line1<br>line2<br>line3</p>"
+    );
+  });
+
+  it("splits paragraphs on blank lines", () => {
+    expect(plainTextToComposerBody("first\n\nsecond\nthird")).toBe(
+      "<p>first</p><p>second<br>third</p>"
+    );
+  });
+});