Fill placeholders when a template is inserted

Templates were a fixed subject and body, so anything that changed per
message -- who it is going to, today's date -- had to be typed over
afterwards.

Eight names are recognised: recipientName, recipientFirstName,
recipientEmail, myName, myEmail, subject, date and time. Dates and times
go through datetime.ts rather than toLocaleDateString, so a template
follows the date order and clock the app was already told to use.

Filling happens on insert rather than on send. What a placeholder came to
is then visible in the composer and can be edited, instead of the message
changing between writing it and sending it.

Two things are deliberately left alone. A placeholder that cannot be
answered yet -- a recipient's name on a draft nobody has addressed -- stays
in the body as written, because substituting an empty string produces
"Hi ,", which is wrong rather than visibly unfinished; leaving the name
says which word is still missing and can be typed over. And a name that is
not a placeholder is left as written too, since a body that quietly ate an
unrecognised token would be worse than one that shows it.

Values are escaped on the way into HTML: a display name comes from a
contact card or a typed address and is not trusted markup.

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

@@ -0,0 +1,80 @@
+import { describe, expect, it } from "vitest";
+import { fillPlaceholders, PLACEHOLDER_NAMES, type PlaceholderContext } from "@/lib/templatePlaceholders";
+
+const AT = new Date("2026-03-04T15:07:00Z");
+
+function ctx(over: Partial<PlaceholderContext> = {}): PlaceholderContext {
+  return {
+    to: [{ name: "Ada Lovelace", email: "[email protected]" }],
+    from: { name: "Grace Hopper", email: "[email protected]" },
+    subject: "Quarterly report",
+    now: AT,
+    ...over,
+  };
+}
+
+describe("fillPlaceholders", () => {
+  it("fills the names it knows", () => {
+    expect(fillPlaceholders("Hi {{recipientFirstName}},", ctx(), { html: true })).toBe("Hi Ada,");
+    expect(fillPlaceholders("{{recipientName}} <{{recipientEmail}}>", ctx(), { html: false })).toBe("Ada Lovelace <[email protected]>");
+    expect(fillPlaceholders("-- {{myName}}", ctx(), { html: true })).toBe("-- Grace Hopper");
+    expect(fillPlaceholders("Re: {{subject}}", ctx(), { html: false })).toBe("Re: Quarterly report");
+  });
+
+  it("tolerates spaces inside the braces but not a different case", () => {
+    expect(fillPlaceholders("{{ myEmail }}", ctx(), { html: false })).toBe("[email protected]");
+    expect(fillPlaceholders("{{MyEmail}}", ctx(), { html: false })).toBe("{{MyEmail}}");
+  });
+
+  it("leaves a placeholder it cannot answer exactly as written", () => {
+    // The case the design is about: a template inserted before the message is
+    // addressed. "Hi ," would be wrong; "Hi {{recipientFirstName}}," is unfinished.
+    const unaddressed = ctx({ to: [] });
+    expect(fillPlaceholders("Hi {{recipientFirstName}},", unaddressed, { html: true })).toBe("Hi {{recipientFirstName}},");
+    expect(fillPlaceholders("{{recipientEmail}}", unaddressed, { html: false })).toBe("{{recipientEmail}}");
+    expect(fillPlaceholders("{{myName}}", ctx({ from: null }), { html: false })).toBe("{{myName}}");
+  });
+
+  it("leaves a name it does not know alone rather than eating it", () => {
+    expect(fillPlaceholders("{{nonsense}} {{}} {{ }}", ctx(), { html: true })).toBe("{{nonsense}} {{}} {{ }}");
+  });
+
+  it("falls back to the local part when a recipient has no name", () => {
+    const c = ctx({ to: [{ name: null, email: "[email protected]" }] });
+    expect(fillPlaceholders("{{recipientName}}", c, { html: false })).toBe("ada.lovelace");
+    expect(fillPlaceholders("{{recipientFirstName}}", c, { html: false })).toBe("ada.lovelace");
+  });
+
+  it("escapes a substituted value on the way into HTML, and not into a subject", () => {
+    const c = ctx({ to: [{ name: 'Ada <script>alert("x")</script>', email: "[email protected]" }] });
+    expect(fillPlaceholders("{{recipientName}}", c, { html: true })).not.toContain("<script>");
+    expect(fillPlaceholders("{{recipientName}}", c, { html: true })).toContain("&lt;script&gt;");
+    expect(fillPlaceholders("{{recipientName}}", c, { html: false })).toContain("<script>");
+  });
+
+  it("repeats a placeholder as many times as it appears", () => {
+    expect(fillPlaceholders("{{recipientFirstName}} {{recipientFirstName}}", ctx(), { html: true })).toBe("Ada Ada");
+  });
+
+  it("answers date and time from the injected clock", () => {
+    const date = fillPlaceholders("{{date}}", ctx(), { html: false });
+    const time = fillPlaceholders("{{time}}", ctx(), { html: false });
+    expect(date).not.toBe("{{date}}");
+    expect(date).toMatch(/2026/);
+    expect(time).not.toBe("{{time}}");
+    expect(time).toMatch(/\d/);
+  });
+
+  it("names every resolver in the list Settings shows", () => {
+    expect(PLACEHOLDER_NAMES).toEqual([
+      "recipientName",
+      "recipientFirstName",
+      "recipientEmail",
+      "myName",
+      "myEmail",
+      "subject",
+      "date",
+      "time",
+    ]);
+  });
+});