PUBLIC_URL comes back, and gen:index takes it as an argument

Removing PUBLIC_URL earlier today was wrong, and the Build section is where it
would have surfaced: gen-index.ts exits 1 without it, so `bun gen:index` fails,
index.gen.html is never written, and `bun start` has no page to serve.

It came out because origin validation was being discontinued — but that was one
of four consumers and the only one that is gone. gen-index needs it for
OpenGraph tags, which crawlers fetch standalone and cannot resolve relative;
task-api-env builds OFFICER_API_HOST from it; and dav/router hard-requires it,
https only, to build an iOS profile.

It is also the one value this machine genuinely cannot derive, which is what
separates it from DATA_PATH and the rest that left today.

gen:index now takes a URL as its first argument, ahead of the environment and
.env: `bun gen:index https://officer.example.com`. Changing the public address is
one command rather than an edit plus a regenerate, and a second address can be
generated for without touching the install's .env.

It also validates now. A relative or scheme-less value substituted silently and
produced OpenGraph tags nothing can resolve — invisible until someone shares a
link and the preview comes back blank.

.env is PORT, PUBLIC_URL, POSTGRES_URL. Verified by running the section; all
three paths through gen:index exercised (absent, valid argument, invalid).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/setup/officer-setup.sh

@@ -462,6 +462,7 @@ if ! skip; then
 
   # Read back before anything is asked; existing values become the defaults.
   ENV_PORT="$(env_get PORT)"
+  ENV_PUBLIC_URL="$(env_get PUBLIC_URL)"
 
   if env_exists; then
     echo "     exists — its values are the defaults below"
@@ -473,9 +474,18 @@ if ! skip; then
   echo ""
   ask_required ENV_PORT "Port Officer listens on" "${ENV_PORT:-9000}"
 
+  echo ""
+  echo "     PUBLIC_URL is where Officer is reached from a browser. It is the one"
+  echo "     thing this machine cannot work out for itself, and three things need"
+  echo "     it: the OpenGraph tags baked into the page by 'bun gen:index', the"
+  echo "     host the task API hands to scripts, and the CalDAV profile an iPhone"
+  echo "     installs — that last one requires https."
+  ask_required ENV_PUBLIC_URL "Public URL" "${ENV_PUBLIC_URL:-http://localhost:${ENV_PORT}}"
+
   echo ""
   echo "     to write:"
   echo "       PORT=${ENV_PORT}"
+  echo "       PUBLIC_URL=${ENV_PUBLIC_URL}"
   echo "       POSTGRES_URL=${POSTGRES_URL%%:*}://…"
   echo ""
   echo "     the install root is not written here — the platform derives it as the"