fix(battleship): no shots until both fleets are deployed

[Epixx1337]

The first player to deploy could fire on an opponent who was still shuffling
their board.

Nothing server-side knew what "ready" meant. The session was created with
turn = sides[1] the instant the lobby started, and the only gate on a move
was g.turn ~= mySide. Board placement lived entirely in the client:
confirmPlacement() flipped its own phase to 'firing' and set
myTurn = humanSide === '1' without telling anyone, so player 1 could shoot
immediately, and player 2 ate a buffered shot the moment they finished
shuffling - with their layout fixed after player 1's.

Move the readiness handshake to the server, which is the only place both
clients agree:

• Sessions carry ready = {}, and games opt in with requiresSetup
  (battleship only - chess/connectfour have no setup phase, and wordle
  bypasses turn logic via freeRelay, so both are unaffected).
• New games:setupReady callback marks a side ready, tells the other side
  ('oppReady'), and once BOTH are in, picks who moves first and pushes
  'begin' to both clients.
• The move handler rejects everything before that with "Opponent is still
  deploying", so the client gate can no longer be bypassed.

Client: confirmPlacement() now reports ready and enters a new 'waiting'
phase instead of starting the match itself; firing begins only on 'begin',
which is also what decides myTurn. The waiting state shows "Waiting for X to
deploy…". The pre-existing pendingShot buffer is kept purely as a safety net

• with the gate in place a shot can no longer arrive early.

Not verified in-game (no FiveM server here); reasoned through the engine and
typechecked. Worth a two-player smoke test before merge.

Fixes #23