default PUBLIC_URL to the tailnet address, not localhost

localhost is wrong on a machine with a tailnet, and quietly so: it works from the
machine itself and nowhere else, so the mistake surfaces on the first phone
rather than during setup. And PUBLIC_URL is not decoration — gen:index bakes it
into the page's OpenGraph tags, the task API hands it to scripts as
OFFICER_API_HOST, and the CalDAV profile builder refuses without it.

The tailnet is where Officer is actually reached, and it is the perimeter the
whole security model rests on now that origin checking is gone. Its address is
the honest default.

Prefers the MagicDNS name over the raw 100.x address — both work, but the name
survives a node being re-registered and is something a person can type. Falls
back to localhost with no tailnet, which is right rather than merely tolerable:
a machine with no private network has no better address to guess.

Suggests http://officer-dev.ts.pastilhas.dev:9000 on this machine.

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

scripts/setup/officer-setup/lib/env.sh

@@ -76,3 +76,43 @@ ENVF
   chmod 600 "$dest"
   return 0
 }
+
+# -----------------------------------------------------------------------------
+# A sensible default for PUBLIC_URL
+# -----------------------------------------------------------------------------
+#
+# localhost is the wrong default on a machine with a tailnet, and quietly so:
+# it works from the machine itself and from nowhere else, so the mistake shows up
+# on the first phone, not during setup.
+#
+# The tailnet is where Officer is actually reached — it is the perimeter the
+# whole security model rests on — so its address is the honest default.
+#
+# MagicDNS name preferred over the raw 100.x address. Both work, but the name
+# survives the node being re-registered and reads as something a person can type,
+# and PUBLIC_URL ends up baked into the page's OpenGraph tags by `bun gen:index`.
+#
+# Falls back to localhost when there is no tailnet, which is correct rather than
+# merely tolerable: a machine with no private network has no other address that
+# is any better a guess.
+tailnet_hostname() {
+  local dns ip
+  dns="$(tailscale status --json 2>/dev/null | grep -oP '"DNSName":\s*"\K[^"]+' | head -1)"
+  dns="${dns%.}"
+  if [[ -n "$dns" ]]; then
+    echo "$dns"
+    return 0
+  fi
+  ip="$(tailscale ip -4 2>/dev/null | head -1)"
+  [[ -n "$ip" ]] && echo "$ip"
+}
+
+default_public_url() {
+  local host
+  host="$(tailnet_hostname)"
+  if [[ -n "$host" ]]; then
+    echo "http://${host}:${1}"
+  else
+    echo "http://localhost:${1}"
+  fi
+}