Read a Markdown file as the document it is

A .md previewed as its own source, which is reading the punctuation
rather than the notes. It now opens rendered, with Rendered | Source in
the dialog footer for anyone who wants what the file actually says.
Markdown only; a .txt has nothing to toggle between.

Rendering is `marked`, sanitised by DOMPurify -- the one the app already
carries for mail. Markdown is not a safe subset of anything: raw HTML
passes through it by design, so a <script> in a file somebody uploaded or
shared into the account is a script tag unless something takes it out.

Images become links rather than pictures. An image in a Markdown file is
either a relative path, which has no base to resolve against here, or a
URL somewhere else, which fetches on open and tells that server the file
was read -- the tracking pixel this app blocks in mail. The link keeps
the alt text and the address, so nothing vanishes silently.

Fixes the PDF preview while here, which never worked: securityHeaders
put X-Frame-Options: DENY on every response including the blob route, so
the iframe showed Chrome's "refused to connect" where the file should
have been -- in Files today and in mail attachments long before that.
The middleware now leaves a header the route has set, and a PDF served
inline says SAMEORIGIN. Nothing else on the server is framable.

server/src/app.test.ts

@@ -88,3 +88,22 @@ test("a Sieve script larger than a compressing hop's threshold survives the prox
     origin.close();
   }
 });
+
+test("only a PDF blob may be framed, and only by us", async () => {
+  /*
+   * The PDF preview is an iframe, and the blanket X-Frame-Options: DENY on
+   * every response blocked it -- the dialog showed Chrome's "refused to
+   * connect" where the file should have been. The middleware now leaves a
+   * header a route has already set, so this pins both halves: the exception
+   * exists, and it did not become the rule.
+   */
+  const app = createApp();
+  const health = await app.request("/api/health");
+  assert.equal(health.headers.get("x-frame-options"), "DENY");
+
+  const { securityHeadersFor } = await import("./app.js");
+  assert.equal(securityHeadersFor("application/pdf", true), "SAMEORIGIN");
+  assert.equal(securityHeadersFor("application/pdf", false), "DENY");
+  assert.equal(securityHeadersFor("image/png", true), "DENY");
+  assert.equal(securityHeadersFor("text/html", true), "DENY");
+});