balance(achievements): strictly-escalating milestone coin rewards

The old reward formula rounded (40+6g)/50 on the raw goal, so adjacent
milestones could pay the same (e.g. "1 win" and "5 wins" both 50) and the curve
was lumpy — not a standard escalating-reward ladder. Reward now scales by tier
index: 50, 150, 250 … capped at 1500, strictly increasing per milestone.
Mirrored client (gamification.ts) + server (Gamification.cs) so live grants match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/online/gamification.ts

@@ -253,12 +253,13 @@ function tier(
   faDesc: (g: string) => string,
   enDesc: (g: number) => string
 ): AchievementDef[] {
-  return goals.map((g) => ({
+  return goals.map((g, i) => ({
     id: `${prefix}_${g}`,
     category,
     metric,
     goal: g,
-    coinReward: Math.min(1500, Math.max(50, Math.round((40 + g * 6) / 50) * 50)),
+    // Reward escalates strictly per milestone tier (50, 150, 250 … capped 1500).
+    coinReward: Math.min(1500, 50 + i * 100),
     icon,
     nameFa: faName(faNum(g)),
     nameEn: enName(g),