Quote images through the proxy, and unproxy them on the way out (#412) (#413)

Reading a message fetches its remote images through this server, so the
sender learns nothing about the reader. Quoting the same message into a
reply fetched them directly: same pixel, same reader, but the request
carried their IP and user agent -- exactly what the proxy withholds.

A quote now proxies them the way the message view does. That alone would
be wrong, because a proxied URL belongs to this deployment: sent
unchanged it would reach the recipient as images only this server can
serve, broken for them and a beacon back here. So buildEmailObject turns
them back into the addresses they came from, beside the pass that
restores images blocked under pr411 and the one that turns editor blob
URLs into cid: references.

Deployments with the proxy off are unaffected: the quote fetches
directly, as reading does there.

Three tests from pr411 asserted the address sat in src when images were
allowed, which was the old behaviour; they now ask whether the draft
fetches it at all, proxied or not.

No new strings.

web/src/store/__tests__/quote-image-policy.test.ts

@@ -30,6 +30,13 @@ const MESSAGE = {
 
 const IDENTITIES = [{ id: "i1", name: "John", email: "[email protected]", replyTo: null }] as unknown as Identity[];
 
+/**
+ * Whether the draft will actually load the image. Allowed images go through
+ * the server's proxy where the deployment has one (#412), so the address is
+ * escaped inside an `/api/image` URL rather than sitting in `src` as it is.
+ */
+const fetched = (html: string) => html.includes(`/api/image?url=${encodeURIComponent(PIXEL)}`) || html.includes(`src="${PIXEL}"`);
+
 function replyDraft() {
   useMail.setState({
     accountId: "a1",
@@ -70,18 +77,18 @@ describe("quoting a message whose images were not allowed", () => {
   it("fetches them once the reader has shown images on that message", async () => {
     useMail.setState({ imagesShown: { m1: true } });
     const d = await replyDraft();
-    expect(d.html).toContain(`src="${PIXEL}"`);
+    expect(fetched(d.html)).toBe(true);
     expect(d.html).not.toContain("data-ihm-blocked");
   });
 
   it("fetches them when the policy is to show images always", async () => {
     useSettings.setState((s) => ({ settings: { ...s.settings, imagePolicy: "always" } }));
-    expect((await replyDraft()).html).toContain(`src="${PIXEL}"`);
+    expect(fetched((await replyDraft()).html)).toBe(true);
   });
 
   it("fetches them from a sender the reader trusts", async () => {
     useSettings.setState((s) => ({ settings: { ...s.settings, trustedImageSenders: ["[email protected]"] } }));
-    expect((await replyDraft()).html).toContain(`src="${PIXEL}"`);
+    expect(fetched((await replyDraft()).html)).toBe(true);
   });
 
   it("leaves them blocked for a stranger when the policy is contacts only", async () => {