Force-close accepted connections and drain session goroutines on shutdown

Canceling the server context closed only the listeners: sessions and
file transfers never observed cancellation, so their goroutines kept
running after ListenAndServe returned, blocked in reads until the
remote side went away. Nothing joined them either, so shutdown raced
whatever work was still in flight.

The server now tracks every accepted connection (sessions and file
transfers) in a registry. After the serve loops stop, ListenAndServe
force-closes the tracked connections, which unblocks their read loops,
and waits on a WaitGroup covering every session, file transfer, and
client writer goroutine before returning. Connections that race in
after shutdown begins are closed on arrival. The registry initializes
lazily so test-constructed Servers keep working.

The 3-second Windows close workaround in handleFileTransfer is skipped
when the context is canceled so in-flight transfers do not delay
process exit, and the transfer goroutine no longer assigns its error
to the accept loop's captured variable.

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