ihasmail 2.0: rebuild as Stalwart-first JMAP webmail

Replace the FastAPI/HTMX prototype with a Node/Hono session proxy and a
React 19/Vite SPA. Mail (conversation view, search operators, labels,
sanitised HTML, privacy image proxy, invites, undo send, templates),
calendar (month/week/day/agenda, invites, free/busy, categories,
context menus), contacts (JSContact, groups, vCard), files, Sieve filter
builder (incl. filter-from-message with retroactive apply), vacation,
identities with default + Reply-To, PWA/mobile layout, push via SSE,
in-memory mock Stalwart for dev, Docker + CI.

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

@@ -0,0 +1,23 @@
+import { describe, expect, it } from "vitest";
+import { formatDuration, parseDuration, zonedToDate, dateToZonedLocal, monthGrid } from "../dates";
+
+describe("dates", () => {
+  it("parses and formats ISO durations", () => {
+    expect(parseDuration("PT1H30M")).toBe(5400);
+    expect(parseDuration("P1DT2H")).toBe(93600);
+    expect(parseDuration("-PT15M")).toBe(-900);
+    expect(formatDuration(5400)).toBe("PT1H30M");
+    expect(formatDuration(-600)).toBe("-PT10M");
+    expect(formatDuration(86400)).toBe("P1D");
+  });
+  it("converts zoned local times to instants", () => {
+    const d = zonedToDate("2024-07-01T12:00:00", "America/New_York");
+    expect(d.toISOString()).toBe("2024-07-01T16:00:00.000Z");
+    expect(dateToZonedLocal(d, "Europe/Berlin")).toBe("2024-07-01T18:00:00");
+  });
+  it("builds a 42-day month grid starting on week start", () => {
+    const g = monthGrid(new Date(2024, 1, 15), 1);
+    expect(g).toHaveLength(42);
+    expect(g[0]!.getDay()).toBe(1);
+  });
+});