Solo watching and a clearer offline start

Putting anything up offline starts a solo session: an in-app server bound
to 127.0.0.1 only, joined like hosting, so tracking, next episode and the
player all work. Invite friends re-hosts on the Host card's port and
password with the same media and position; Stop needs no confirmation.
Offline, the Library opens with a welcome band (Join a friend, Host, Watch
solo); without a Jellyfin account it leads with Files and Link, Jellyfin
is an optional card. Play buttons work offline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/HOW-IT-WORKS.md

@@ -44,7 +44,13 @@ A five-minute tour. Details: [ARCHITECTURE.md](ARCHITECTURE.md), wire format:
 - **Dedicated server**: `watch-together-server`, a separate program in the
   release tarball, for an always-on box. See the README.
 
-Friends join either way with Join in the strip at the top of the window.
+Friends join either way with Join a friend in the strip at the top of the
+window; guests need no Jellyfin or account.
+
+- **Watch solo**: put something up without joining or hosting and the
+  app hosts a private server reachable only from this computer, so
+  watching alone works like a session (progress, next episode). Invite
+  friends turns it into a normal hosted server, video and position kept.
 
 ## One session, start to finish