fix(nano): -Dmojang.sessionserver takes the full hasJoined URL, not the base

d417efc got this backwards, in both the code comment and the installer summary.
It claimed authlib appends /session/minecraft/hasJoined itself, so the property
should be given the base URL only. Velocity does not work that way, and a real
login says so: pointed at http://127.0.0.1:8081, a Mojang login arrives at nano
as

    GET /?username=FLYEMOJ1&serverId=-23ae0b50...

with no path at all. Velocity appends the query string to the property verbatim
and issues the request itself; authlib is not in the loop. nano has no route on
/, so it answers 404 and Velocity kicks the player with authservers_down.
Velocity's own default for the property is the full URL,
https://sessionserver.mojang.com/session/minecraft/hasJoined, which is the same
thing said another way.

With the full endpoint URL the same account logs straight in, so both the
nano.go header and summary_nano now print

    -Dmojang.sessionserver=http://127.0.0.1:8081/session/minecraft/hasJoined

and note that the flag belongs between `java` and `-jar`.

Verified against Velocity 3.5.1 + Paper 26.2 on the deploy host: a Mojang login
reaches the backend with its real Mojang UUID unchanged, and a LittleSkin login
under the same username reaches it as UUIDv3(felisAuthNS, "littleskin:"+id) --
two different players on the backend, which is the point.

deploy/bootstrap.sh

@@ -1247,9 +1247,10 @@ summary_nano() {
   systemctl --no-pager --full status felis-nano 2>/dev/null | head -n 6 || true
   echo
   log "hasJoined endpoint: http://${host}:${port}/session/minecraft/hasJoined"
-  log "Point Velocity at it — add to the proxy JVM startup flags:"
-  log "    -Dmojang.sessionserver=http://${host}:${port}"
-  log "    (base URL only — authlib appends the path itself)"
+  log "Point Velocity at it — add to the proxy JVM startup flags (between java and -jar):"
+  log "    -Dmojang.sessionserver=http://${host}:${port}/session/minecraft/hasJoined"
+  log "    (the FULL endpoint URL, path included — Velocity's default for this property is"
+  log "     the full https://sessionserver.mojang.com/session/minecraft/hasJoined)"
   if nano_listen_is_loopback; then
     log "Bound to loopback: reachable from Velocity on THIS host, and from nowhere else."
     log "Proxy on another machine? Re-run with FELIS_NANO_LISTEN=<private-ip>:${port} and"