Ask before opening a shared item in a message

The share address takes a plain form POST, which any website can make,
and the app opened whatever arrived straight into a composer. It now
shows what was shared -- the title, the start of the text and link, and
the file names -- and opens a message only when the reader chooses to.
Discarding drops it.

Confirm dialogs now put a message that is not plain text in a div, since
the summary has blocks of its own.

Three new strings, translated in all nine catalogs.

web/src/lib/__tests__/shareTarget.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
-import { collectShare, shareBody, SHARE_MAX_AGE_MS } from "@/lib/shareTarget";
+import { shareSummary, collectShare, shareBody, SHARE_MAX_AGE_MS } from "@/lib/shareTarget";
 import { SW_CACHE_NAME } from "@/lib/sw/swCache";
 
 /**
@@ -112,3 +112,19 @@ describe("the body a share turns into", () => {
     expect(shareBody({ text: "a thought", url: "" })).toBe("a thought");
   });
 });
+
+describe("shareSummary", () => {
+  const file = (name: string) => new File(["x"], name);
+  it("gives the title, the text and link together, and the file names", () => {
+    expect(shareSummary({ title: " Trip ", text: "See this", url: "https://example.com", files: [file("a.jpg")] })).toEqual({
+      title: "Trip",
+      preview: "See this https://example.com",
+      files: ["a.jpg"],
+    });
+  });
+  it("shortens a long text rather than showing all of it", () => {
+    const { preview } = shareSummary({ title: "", text: "word ".repeat(100), url: "", files: [] });
+    expect(preview.length).toBeLessThanOrEqual(160);
+    expect(preview.endsWith("…")).toBe(true);
+  });
+});

web/src/lib/shareTarget.ts

@@ -117,3 +117,17 @@ export function shareBody(share: Pick<SharedContent, "text" | "url">): string {
   if (!url || text.includes(url)) return text;
   return text ? `${text}\n\n${url}` : url;
 }
+
+/**
+ * What a share holds, in the few words the confirmation shows.
+ *
+ * Only what the reader needs to recognize it as theirs: the title, the start
+ * of the text or link, and the names of the files. It is shown before any of
+ * it goes near a message, because the page cannot tell a share the reader
+ * made from one a website posted at the same address (#375 review).
+ */
+export function shareSummary(share: SharedContent): { title: string; preview: string; files: string[] } {
+  const body = [share.text, share.url].map((s) => s.trim()).filter(Boolean).join(" ");
+  const preview = body.length > 160 ? `${body.slice(0, 157).trimEnd()}…` : body;
+  return { title: share.title.trim(), preview, files: share.files.map((f) => f.name) };
+}