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.

Caddyfile.example

@@ -0,0 +1,9 @@
+# Example reverse proxy (Caddy) in front of ihasmail.
+# TLS is automatic. ihasmail sets Secure cookies and HSTS when X-Forwarded-Proto is https.
+mail.example.com {
+    encode zstd gzip
+    reverse_proxy 127.0.0.1:8080 {
+        # Keep SSE (push) connections open
+        flush_interval -1
+    }
+}