feat(hint): unify the vs_ai idle hint online + offline (server-enforced, monotonic)

Online vs_ai hints were broken: #207 made the vs_ai hint button always-enabled and
wallet-free (assuming all vs_ai = the offline idle-gate), but the backend still served
online vs_ai from the allowance/wallet, so over-clicking hit ErrNoHintsLeft -> a generic
error toast. Now online vs_ai uses the SAME idle-gate model as offline.

Backend: Hint() for a vs_ai game skips the allowance/wallet and increments no hints_used
(owner: vs_ai counts toward no hint statistic), and is idle-gated from the SERVER clock --
it returns ErrHintLocked (code hint_locked) until the robot's last move + 30 min, else
serves the top move. GameState/StateView expose hint_unlock_left_seconds (server-computed
seconds remaining; 0 for a human game / first move / not-your-turn). Pure helper
hintUnlockLeftSeconds unit-tested.

Wire: StateView gains hint_unlock_left_seconds (FlatBuffers, additive); pkg/wire + gateway
transcode carry it (round-trip test).

Client: the gate counts down from a MONOTONIC clock (performance.now()) anchored to the
source's seconds-left when it lands (on load from the view; to the full window when the
robot moves), so a client clock change cannot skew it and a relaunch re-reads a fresh
value. The vs_ai hint button (online + offline) shows the lock + toast; doHint handles the
hint_locked backstop by re-syncing. Replaces #207's absolute hintUnlockAtMs on the
wire/model/delta (the offline record keeps the absolute for persistence; the view exposes
seconds-left).

Docs FUNCTIONAL(+_ru)/ARCHITECTURE updated. Verified: go build/vet + game/server/transcode
tests (+ new hintUnlockLeftSeconds); ui check 0 / unit 490 / e2e 198 (one pre-existing
webkit offline flake) / app entry 114.2/115.

ui/src/gen/fbs/scrabblefb/state-view.ts

@@ -74,8 +74,13 @@ walletBalance():number {
   return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
 }
 
+hintUnlockLeftSeconds():number {
+  const offset = this.bb!.__offset(this.bb_pos, 18);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
 static startStateView(builder:flatbuffers.Builder) {
-  builder.startObject(7);
+  builder.startObject(8);
 }
 
 static addGame(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset) {
@@ -130,12 +135,16 @@ static addWalletBalance(builder:flatbuffers.Builder, walletBalance:number) {
   builder.addFieldInt32(6, walletBalance, 0);
 }
 
+static addHintUnlockLeftSeconds(builder:flatbuffers.Builder, hintUnlockLeftSeconds:number) {
+  builder.addFieldInt32(7, hintUnlockLeftSeconds, 0);
+}
+
 static endStateView(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;
 }
 
-static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number, alphabetOffset:flatbuffers.Offset, walletBalance:number):flatbuffers.Offset {
+static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offset, seat:number, rackOffset:flatbuffers.Offset, bagLen:number, hintsRemaining:number, alphabetOffset:flatbuffers.Offset, walletBalance:number, hintUnlockLeftSeconds:number):flatbuffers.Offset {
   StateView.startStateView(builder);
   StateView.addGame(builder, gameOffset);
   StateView.addSeat(builder, seat);
@@ -144,6 +153,7 @@ static createStateView(builder:flatbuffers.Builder, gameOffset:flatbuffers.Offse
   StateView.addHintsRemaining(builder, hintsRemaining);
   StateView.addAlphabet(builder, alphabetOffset);
   StateView.addWalletBalance(builder, walletBalance);
+  StateView.addHintUnlockLeftSeconds(builder, hintUnlockLeftSeconds);
   return StateView.endStateView(builder);
 }
 }