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.

cmd/felis/nano.go

@@ -5,11 +5,12 @@ package main
 // full Felis control plane (no k3s, no Postgres, no DB). It reads [[auth_source]] from
 // felis.toml, leads with Mojang as the code-owned identity anchor (正版优先), and serves the
 // vanilla sessionserver hasJoined endpoint. Point Velocity at it with
-//   -Dmojang.sessionserver=http://127.0.0.1:8081
-// — the base URL only: authlib appends /session/minecraft/hasJoined itself. Then it
-// verifies logins against Mojang plus every configured third-party source. Serving a
-// proxy on another host means binding off-loopback with -listen; see the flag below for
-// why that is an explicit opt-in and not the default.
+//   -Dmojang.sessionserver=http://127.0.0.1:8081/session/minecraft/hasJoined
+// — Velocity's property takes the FULL endpoint URL, path included (its default is the
+// full https://sessionserver.mojang.com/session/minecraft/hasJoined), and Velocity issues
+// that request itself rather than through authlib. Then it verifies logins against Mojang
+// plus every configured third-party source. Serving a proxy on another host means binding
+// off-loopback with -listen; see the flag below for why that is an explicit opt-in.
 //
 // This is the no-database delivery of the identical brain `felis api` mounts through its
 // route table (internal/api.HasJoinedHandler). `felis setup --nano` / the bootstrap nano